Highly efficient compact head-mounted display system

ABSTRACT

There is provided an optical device, including a light-transmitting substrate having at least two parallel major surfaces, edges and an output aperture, an optical element for coupling light waves into the substrate to effect total internal reflection, a plurality of redirecting elements positioned outside of the substrate comprising at least two spaced-apart redirecting elements having a selectable laterally displaceable reflection-transmission ratio, and at least one reflecting surface having at least one active side located between the two major surfaces of the light-transmitting substrate for coupling light waves out of the substrate, wherein light waves trapped inside the substrate are coupled out from the substrate through the output aperture substantially inclined in relation to the normal to the substrate major surfaces, and are reflected from the redirecting elements into a viewer&#39;s eye, and wherein at least one of the redirecting elements is lateral displaceable in relation to another redirecting element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/753,170, filed Apr. 2, 2020 for “HIGHLY EFFICIENT COMPACT HEAD-MOUNTDISPLAY SYSTEM”, which are all hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to substrate-based light waves guidedoptical devices, and particularly to devices which include a reflectingsurface carried by a light-transmissive substrate and a dynamicpartially reflecting surface which is attached the substrate.

The invention can be implemented to advantage in a large number ofimaging applications, such as, head-mounted and head-up displays,cellular phones, compact displays, 3-D displays, compact beam expanders,as well as non-imaging applications such as flat-panel indicators,compact illuminators and scanners.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is inhead-mounted displays (HMDs), wherein an optical module serves both asan imaging lens and a combiner, in which a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM), such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), or a scanning source andsimilar devices, or indirectly, by means of a relay lens, or an opticalfiber bundle. The display comprises an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theviewer by means of a reflecting or partially reflecting surface actingas a combiner for non-see-through and see-through applications,respectively. Typically, a conventional, free-space optical module isused for these purposes. As the desired field-of-view (FOV) of thesystem increases, such a conventional optical module becomes larger,heavier and bulkier, and therefore, even for a moderate performancedevice, is impractical. This is a major drawback for all kinds ofdisplays but especially in HMDs, wherein the system should be as lightand compact as possible.

The need for compactness has led to several different complex opticalsolutions, all of which, on the one hand, are still not sufficientlycompact for most practical applications, and on the other hand, suffermajor drawbacks in terms of manufacturability. Furthermore, theeye-motion-box (EMB) of the optical viewing angles resulting from thesedesigns is usually very small, typically less than 8 mm. Hence, theperformance of the optical system is very sensitive, even for smallmovements of the optical system relative to the eye of the viewer, anddo not allow sufficient pupil motion for conveniently reading text fromsuch displays.

The teachings included in Publication Nos. WO2017/141239, WO2017/141240,and WO2017/141242, are herein incorporated by reference.

SUMMARY OF THE INVENTION

The present invention facilitates the provision of compact substratesfor, amongst other applications, HMDs. The invention allows relativelywide FOVs together with relatively large EMB values. The resultingoptical system offers a large, high-quality image, which alsoaccommodates large movements of the eye. The optical system according tothe present invention is particularly advantageous because it issubstantially more compact than state-of-the-art implementations, andyet it can be readily incorporated, even into optical systems havingspecialized configurations.

A further application of the present invention is to provide a compactdisplay with a wide FOV for mobile, hand-held applications such ascellular phones. In today's wireless internet-access market, sufficientbandwidth is available for full video transmission. The limiting factorremains the quality of the display within the device of the end-user.The mobility requirement restricts the physical size of the displays,and the result is a direct-display with poor image viewing quality. Thepresent invention enables a physically compact display with a largevirtual image. This is a key feature in mobile communications, andespecially for mobile internet access, solving one of the mainlimitations for its practical implementation, thereby enabling theviewing of digital content of a full format internet page within asmall, hand-held device, such as a cellular phone.

A broad object of the present invention is, therefore, to alleviate thedrawbacks of state-of-the-art compact optical display devices and toprovide other optical components and systems having improvedperformance, according to specific requirements.

In accordance with the present invention there is therefore provided anoptical device comprising an input aperture, an output aperture, alight-transmitting substrate having at least two major surfaces andedges, composed of a first optical material, a coupling-in elementpositioned outside of the substrate and composed of a second opticalmaterial, for coupling light waves having a field-of view into thesubstrate, a first flat reflecting surface located between the two majorsurfaces of the light-transmitting substrate for reflecting thecoupled-in light waves 1:0 effect total internal reflection from themajor surfaces of the substrate, a second flat reflecting surface,having at least one active side located between the two major surfacesof the light-transmitting substrate for coupling light waves out of thesubstrate, and a redirecting optical element positioned outside of thesubstrate for redirecting light waves coupled-out from the substrateinto a viewer's eye, wherein the refractive indices of the first and thesecond optical materials are substantially different and the ratiobetween the field of view of the light waves coupled-out from thesubstrate into the viewers' eye and the field of view of the light wavescoupled inside the substrate, is substantially bigger than therefractive index of the first optical material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in connection with certain preferredembodiments, with reference to the following illustrative figures sothat it may be more fully understood.

With specific reference to the figures in detail, it is stressed thatthe particulars shown are by way of example and for the purpose ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented to provide what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the invention. In this regard, no attempt is madeto show structural details of the invention in more detail than isnecessary for a fundamental understanding of the invention. Thedescription taken with the drawings are to serve as direction to thoseskilled in the art as to how the several forms of the invention may beembodied in practice.

In the drawings:

FIGS. 1A and 1B are a side view of a prior art exemplarylight-transmitting substrate;

FIGS. 2A and 2B illustrate desired reflectance and transmittancecharacteristics of selectively reflecting surfaces, used in a prior artexemplary light-transmitting substrate, for two ranges of incidentangles;

FIG. 3 illustrates a reflectance curve as a function of the incidentangle for an exemplary dielectric coating;

FIG. 4 is a schematic sectional view of a light-transmitting substrate,wherein the coupling-in, as well as the coupling-out elements, arediffractive optical elements;

FIGS. 5A and 5B illustrate sectional views of a transparent substratehaving coupling-in and coupling-out surfaces, and a partially reflectingcombining element;

FIGS. 6A and 6B are schematic sectional views of an active foldingpartially reflecting element which dynamically redirects the coupled-outlight waves into a viewer's eye, according to the present invention;

FIGS. 7A and 7B are other embodiments of schematic sectional views of anactive folding partially reflecting element which dynamically redirectsthe coupled-out light waves into a viewer's eye, according to thepresent invention;

FIGS. 8A, 8B, and 8C illustrate sectional views of a dynamic partiallyreflecting element comprising two identical transferrable arrays ofparallel mirrors, according to the present invention;

FIGS. 9A, 9B, and 9C illustrate sectional views of another dynamicpartially reflecting element comprising three transferrable arrays ofparallel mirror, according to the present invention;

FIGS. 10A and 10B illustrate some optical characteristic of a prior artoptical system during a period of one frame-time;

FIGS. 11E and 11B illustrate some optical characteristic of an opticalsystem during a period of one frame-time, according to the presentinvention;

FIGS. 12A and 12B illustrate some optical characteristic of an opticalsystem during a period of one frame-time, wherein the display source ofthe system is based on a time sequential color imaging, according to thepresent invention;

FIGS. 13A, 13B, and 13C illustrate sectional views of a transparentsubstrate comprising a coupling-in surface, two coupling-out surfaces, adynamic partially reflecting element and dynamic re-coupling surfaces,according to the present invention;

FIGS. 14A, 14B, 14C, and 14D illustrate sectional views of a transparentsubstrate comprising a coupling-in surface, three coupling-out surfaces,a dynamic partially reflecting element, dynamic re-coupling surfaces, aneyeball tracking unit, and a dynamic control unit, according to thepresent invention;

FIGS. 15A and 15B illustrate a dynamic partially reflecting element,comprising an electrically switchable transreflective mirror, pixelizedinto a two-dimensional array of pixels, according to the presentinvention;

FIGS. 16A and 16B illustrate a dynamic partially reflecting element,comprising two identical, two-dimensional arrays of transferrablemirrors, according to the present invention;

FIGS. 17A, 17B, and 17C illustrate sectional views of a transparentsubstrate comprising a coupling-in surface, two coupling-out surfaces, apair of angular sensitive reflecting elements and an array ofredirecting surfaces, according to the present invention;

FIG. 18 schematically illustrates active parts of a coupling-out surfaceaccording to the viewing angle and the eye-motion-box (EMB) of thesystem;

FIGS. 19A and 19B are graphs illustrating the reflection of incidentlight waves on two different angular sensitive coupling-out surfaces asa function of the incident angle, according to the present invention;

FIG. 20 schematically illustrates active parts of redirecting elementsaccording to the viewing angle and the eye-motion-box of the system,wherein at least part of the coupling-out elements are angular sensitivereflecting surfaces;

FIG. 21 illustrates sectional views of a transparent substratecomprising a coupling-in surface, two coupling-out surfaces, an array ofangular sensitive reflecting elements and an array of redirectingsurfaces, according to the present invention;

FIG. 22 illustrates sectional views of a transparent substratecomprising a coupling-in surface, three coupling-out surfaces, an arrayof angular sensitive reflecting elements and an array of redirectingsurfaces, according to the present invention;

FIG. 23 illustrates sectional views of a transparent substratecomprising an angular sensitive coupling-in surface, two coupling-outsurfaces, a pair of partially reflecting elements for coupling the lightwaves out of the substrate and an array of redirecting surfaces,according to the present invention;

FIG. 24A illustrates a sectional view of a transparent substratecomprising an angular sensitive coupling-in surface, two coupling-outsurfaces, a pair of angular sensitive reflecting elements for couplingthe light waves out of the substrate and an array of redirectingsurfaces, according to the present invention;

FIG. 24B illustrates a sectional view of a transparent substratecomprising an angular sensitive coupling-in surface, two coupling-outsurfaces, a pair of angular sensitive reflecting elements for couplingthe light waves out of the substrate and an array of redirectingsurfaces, wherein different parts of the substrate are composed ofdifferent optical materials;

FIGS. 25A, 25B and 25C are graphs illustrating the reflection ofincident light waves on three different angular sensitive surfaces as afunction of the incident angle, according to the present invention;

FIG. 26 is a graph illustrating the brightness efficiency of the lightwaves as a function of the FOV of the system;

FIGS. 27A and 27B illustrate sectional views of a transparent substratecomprising a single coupling-out surface, a redirecting prism and acoupling-in prism, wherein different a parts of the substrate arecomposed of different optical materials;

FIG. 28A is a schematic sectional-view of folding reflecting surfaceswhich redirect the coupled-out light waves into the viewer's eye,according to the present invention;

FIG. 28B is a graph illustrating the brightness of the coupled-out lightwaves as a function of the aperture, according to the present invention;

FIGS. 29A, 29B and 29C are schematic sectional views of a HUD systemcomprising active folding partially reflecting element which dynamicallyredirects the coupled-out light waves into both of the viewer's eyes,according to the present invention;

FIGS. 30A and 30B are schematic sectional top and side views of anotherHUD system wherein the light waves propagate inside the substrate alongthe vertical axis, according to the present invention;

FIGS. 31A, 31B and 31C are schematic sectional views of yet another HUDsystem comprising at least one pair of angular sensitive coupling-outsurfaces and a single flat partially reflecting element which redirectsthe coupled-out light waves into both viewer's eyes, according to thepresent invention;

FIGS. 32A and 328 are graphs illustrating the reflection of incidentlight waves on two different annular sensitive coupling-out surfaces asa function of the incident angle, according to the present invention,and

FIG. 33 schematically illustrates active parts of the single flatredirecting element according to the viewing angle and thehead-motion-box of the system, wherein at least part of the coupling-outelements are angular sensitive reflecting surfaces.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1A illustrates a sectional view of a prior art light-transmittingsubstrate. The first reflecting surface 16 is illuminated by acollimated light wave 12 emanating from a display source 4 andcollimated by a lens 6 located between the source 4 and a substrate 20of the device. The reflecting surface .16 reflects the incident lightfrom the source such that the light wave is trapped inside the planarsubstrate 20, by total internal reflection. After several reflectionsoff the major surfaces 26, 27 of the substrate 20, the trapped lightwaves reach a partially reflective element 22, which couple the lightout of the substrate into the eye 24, having a pupil 25, of a viewer.Herein, the input surface of the substrate will be defined as thesurface through which the input light waves enter the substrate, and theoutput surface of the substrate will be defined as the surface throughwhich the trapped light waves exit the substrate. In the case of thesubstrate illustrated in FIG. 1, both the input and the output surfacescoincide with the lower surface 26. Other configurations are envisioned,however, in which the input and the image light waves from the displacesource 4 are located on opposite sides of the substrate, or on one ofthe edges of the substrate.

The element which couples-out the light waves from the substrate can beeither a single partially reflective surface 22, as illustrated in FIG.1A, or an array of partially reflecting surfaces 22 a, 22 b etc. asillustrated in FIG. 1B. In see-through systems, such as HMDs foraugmented reality (AR) applications, wherein the viewer should see theexternal scene through the substrate, the partially reflecting surfaces22 should be at least partially transparent to enable the external lightrays 33 to pass through the substrate and to reach the viewer's eye 24.The optimal value of the transmissivity of the partially reflectingsurfaces, however, is not a constant and depends on the lightingconditions of the external scene. For bright scenes, in order to improvethe contrast of the projected image, it is required that thereflectivity of the partially reflecting surfaces will be high tomaximize the brightness of the image, while the transmissivity of thesurfaces should be relatively low to prevent the external scene fromdazzling the viewer. On the other band, for dark external scenes, it isrequired that the transmissivity of the surfaces should be relativelyhigh in order not to block the external view. As a result, it would beadvantageous to have an optical system wherein the transmissivity (andconsequently the reflectance) of the partially reflecting surfaces 22can be dynamically controlled, either manually by the viewer, orautomatically by a pre-set mechanism which measures the brightness ofthe external view. Unfortunately, for most of the present technologieswhich are used to materialize see-through augmented reality systems, thepossibility to utilize active partially reflecting surfaces isimpractical.

Referring to the optical embodiment illustrated in FIG. 1B and assumingthat the central light wave of the source is coupled out of thesubstrate 20 in a direction normal to the substrate surface 26, thepartially reflecting surfaces 22 a, 22 b are flat, and the off-axisangle of the coupled light wave inside the substrate 20 is air then theangle α_(sur2) between the reflecting surfaces and the major surfaces ofthe substrate is:

$\begin{matrix}{\alpha_{{sur}\; 2} = {\frac{\alpha_{in}}{2}.}} & (1)\end{matrix}$

As can be seen in FIG. 1B, the trapped rays arrive at the reflectingsurfaces from two distinct directions 28, 30. In this particularembodiment, the trapped rays arrive at the partially reflecting surface22 from one of these directions 28 after an even number of reflectionsfrom the substrate major surfaces 26 and 27, wherein the incident anglepre, between the trapped ray and the normal to the reflecting surfaceis:

$\begin{matrix}{\beta_{ref} = {{\alpha_{in} - \alpha_{{sur}\; 2}} = {\frac{\alpha_{in}}{2}.}}} & (2)\end{matrix}$

The trapped rays arrive at the partially reflecting surface 22 from thesecond direction 30 after an odd number of reflections from thesubstrate surfaces 26 and 27, where the off-axis angle isα′_(in)=−α_(in) and the incident angle between the trapped ray and thenormal to the reflecting surface is:

$\begin{matrix}{{\beta_{ref}^{\prime} = {{\alpha_{in}^{\prime} - \alpha_{{sur}\; 2}} = {{\alpha_{in} - \alpha_{{sur}\; 2}} = {- \frac{3\alpha_{in}}{2}}}}},} & (3)\end{matrix}$

where, the minus sign denotes that the trapped ray impinges on the otherside of the partially reflecting surface 22. As further illustrated inFIG. 1B, for each reflecting surface, each ray first arrives at thesurface from the direction 30, wherein some of the rays again impinge onthe surface from direction 28. In order to prevent undesired reflectionsand ghost images, it is important that the reflectance be negligible forthe rays that impinge on the surface having the second direction 28.

A solution for this requirement that exploits the angular sensitivity ofthin film coatings was previously proposed in the Publications referredto above. The desired discrimination between the two incident directionscan be achieved if one angle is significantly smaller than the otherone. It is possible to provide a coating with very low reflectance athigh incident angles, and a high reflectance for low incident angles.This property can be exploited to prevent undesired reflections andghost images by eliminating the reflectance in one of the twodirections. For example, choosing β_(ref)˜25°, it can be calculatedthat:

β′_(ref)=105°; α_(in)=50°; α′_(in)=130°; α_(sur2)=25°.  (4)

If a reflecting surface is determined for which β′_(ref) is notreflected but β_(ref) is, then the desired condition is achieved.

Referring now specifically to FIGS. 2A and 2B, these figures illustratedesired reflectance behavior of partially reflecting surfaces. While theray 32 (FIG. 2A), having an off-axis angle of β_(ref)˜25°, is partiallyreflected and coupled out of the substrate 20, the ray 36 (FIG. 2B),which arrives at an off-axis angle of β′_(ref)˜75° to the reflectingsurface (which is equivalent to β′_(ref)˜105°, is transmitted throughthe reflecting surface 34, without any notable reflection.

FIG. 3 illustrates the reflectance curve of a typical partiallyreflecting surface of this specific system, as a function of theincident angle for S-polarized light with the wavelength λ=550 nm. For afull-color display, similar reflectance curves should be achieved forall the other wavelengths in the photopic region. There are twosignificant regions in this graph: between 65° and 85°, where thereflectance is very low, and between 10° and 40°, where the reflectanceincreases monotonically with increasing incident angles. As can be seenin FIGS. 2 and 3, the requested reflectance behavior of the partiallyreflective surfaces 22 of the embodiment illustrated in FIGS. 1A and 1Bis not conventional, and indeed, cannot be materialized as an activepartially reflective surface using present technologies. Furthermore,even if such a requested active technology were to be found in thefuture, to keep the low reflectance at the higher angular region, thereflectance at the lower angular region cannot be higher than 20%-30%and hence, the maximum achievable efficiency is comparatively low. As aresult, the idea of utilizing an active partially reflecting surface forthe embodiment illustrated in FIGS. 1A and 1B is impractical.

Another approach to couple light waves into and out from a light-guidedoptical element is by using diffractive elements. As illustrated in FIG.4, the light rays 38 and 40 are coupled into the transparent substrate20 by a diffractive element 48, and after sonic total internalreflection from the external surfaces of the substrate, the light raysare coupled-out from the substrate by a second diffractive element 50.As illustrated, ray 38 is coupled-out at least twice at two differentpoints 52 and 54 on element 54. Consequently, to achieve uniform outputlight waves, the diffraction efficiency of element 50 should beincreased gradually along the ξ axis. It is, however, complicated tomaterialize dynamic gratings using the present techniques, and it ispractically impossible to achieve same for the particularly requestedgrating function of element 50. As a result, it is not possible to applythe idea of utilizing a dynamic element for the diffractive embodimentillustrated in FIG. 4.

FIGS. 5A and 5B illustrate embodiments for overcoming theabove-described problem, according to the present invention. Instead ofusing a single element (22 in FIG. 1A or 50 in FIG. 4), which performsthe dual function of coupling the light waves out of the substrate 20,as well as directing the light waves into the user's eye 24, therequested function is divided into two different elements; namely, oneelement which is embedded. inside the substrate couples the light wavesout of the substrate, while a second. conventional partially reflectingelement which is located out of the substrate, redirects the light wavesinto the viewer's eye. As illustrated in FIG. 5A, two rays 63 (dashedlines) from a plane light wave emanating from a display source andcollimated by a lens (not shown) enter a light transparent substrate 64,having two parallel major surfaces 70 and 72, through the input aperture86 of the coupling-in prism 55, at an incident angle of α_(in) ⁽⁰⁾ withrespect to the major surfaces 70, 72 of the substrate. The rays impingeon the reflecting surface 65, which is inclined at an angle α_(sur1) tothe major surfaces of the substrate. The reflecting surface 65 reflectsthe incident light rays such that the light rays are trapped inside aplanar substrate 64 by total internal reflection from the majorsurfaces. In order to differentiate between the various “propagationorders” of the trapped light waves, a superscript (i) will denote theorder i. The input light waves which impinge on the substrate in thezero order are denoted by the superscript (0). After each reflectionfrom the coupling-in reflecting surface the order of the trapped ray isincreased by one from (i) to (i+I). The off-axis angle α_(in) ⁽¹⁾between the trapped ray and the normal to the major surfaces 70, 72 is

α_(in) ⁽¹⁾=α_(in) ⁽⁰⁾+2·α_(sur1).  (5)

After several reflections off the surfaces of the substrate, the trappedlight rays reach a second flat reflecting surface 67, which couples thelight rays out of the substrate. Assuming that surface 67 is inclined atthe same angle to the major surfaces as the first surface 65, that is tosay, surfaces 65 and 67 are parallel and α_(sur2)=α_(sur1), then theangle α_(out) between the coupled-out rays and the normal to thesubstrate plane is

α_(out)=α_(in) ⁽¹⁾−2·α_(sur2)=α_(in) ⁽¹⁾−2·α_(sur1)=α_(in) ⁽⁰⁾.  (6)

That is to say, the coupled-out light rays are inclined to the substrateat the same angle as the incident light rays. So far, the coupled-inlight waves behave similarly to the light waves illustrated in FIG. 1A.FIG. 5A, however, illustrates, a different behavior wherein two lightrays 68 (dashed-dotted lines), having the same incident angle of α_(in)⁽⁰⁾ as rays 63, impinge on the right side of the reflecting surface 65.After two reflections from surface 65, the light waves are coupledinside the substrate 64 by a total internal reflection, and the off-axisangle of the trapped rays inside the substrate is now

α_(in) ⁽²⁾=α_(in) ⁽¹⁾+2·α_(sur1)=α_(in) ⁽⁰⁾+4·α_(sur1)=α_(in) ⁽⁰⁾.  (7)

After several reflections off the major surfaces of the substrate, thetrapped light rays reach the second reflecting surface 67. The lightrays 68 are reflected twice from the coupling-out surface 67 and arecoupled out from the substrate at the same off-axis angle α_(out) as theother two rays 63 which are reflected only once from surfaces 65 and 67,which is also the same incident input angle of these four rays on thesubstrate major planes.

As illustrated in FIG. 5A, the inclination angle α_(out) of the imagecan be adjusted by adding a partially reflecting surface 79 which isinclined at an angle of

$\frac{\alpha_{in}}{2}$

to the surface 72 of the substrate. As shown, the image is reflected androtated such that it passes again through the substrate substantiallynormal to the substrate major surfaces and reaches the viewer's eye 24through the output aperture 89. To minimize distortion and chromaticaberrations, it is preferred to embed surface 79 in a redirecting prism80, and to complete the shape of the substrate 64 with a second prism82, both of them fabricated of a material similar to that of thesubstrate. In order to minimize the thickness of the system, it ispossible, as illustrated in FIG. 5B, to replace the single reflectingsurface 80 with an array of parallel partially reflecting surfaces 79 a,79 b, etc., where the number of the partially reflecting surfaces can bedetermined according to the requirements of the system.

There are two contradicting requirements from the coupling-out surface67. On the one hand, the first two order images F⁽¹⁾ and F⁽²⁾ should bereflected from that plane, while on the second hand, the zero orderimage F⁽⁰⁾ from the substrate 64 should substantially pass through it,after being reflected from surface 79, with no significant reflections.In addition, for see-through systems, the transparency of the opticalsystem for substantially normal incident light ray 83 from the externalscene should be as high as possible. A possible way to achieve this isto use an air gap in surface 67. For achieving a rigid system, it ispreferred, however, to apply an optical adhesive in surface 67, in orderto cement the substrate 64 with prism 82 using an optical adhesivehaving a refractive index, is which is substantially smaller than thatof the substrate.

An alternative approach is to exploit a moth-eye film, or any similarhyperfine structure, as the required angular sensitive reflectivemechanism. That is to say, when prism 82 is attached to the externalsurface 67 of the substrate 64, an air gap film is cemented to prism 82such that the hyperfine structure faces surface 67 after the attachment.Therefore, when the coupled-in light waves inside the substrates 64impinge on the hyperfine structure at different oblique angles, they“see” only the external part of the periodic structure. The actualrefractive index, which is “seen” by the incoming optical light waves,is therefore close to the refractive index of the air, and the totalinternal reflection mechanism is preserved. On the other hand, the airgap film is substantially transparent to the incoming light waves fromthe external scene 83 or to the light waves which are coupled out fromthe substrate 64 and reflected back by surface 79. In any of theproposed approaches, to minimize the Fresnel reflections of thetransmitted light waves from the coupling-out surface 67, it ispreferred to apply a suitable anti-reflective (AR) coating to thissurface.

As explained above with regard to FIG. 1A, in see-through systems suchas HMDs for augmented reality (AR) applications, wherein the viewershould see the external scene through the substrate, the partiallyreflecting surfaces 79 should be at least partially transparent toenable the external light rays 63 and 68 passing through the substrateand reaching the viewer's eye 24. Since surfaces 79 are only partiallyreflective, only part of the coupled light waves 63 and 68 is reflectedby surfaces 79 and reaches the viewer's eye, while another part of thelight waves 84 passes through surfaces 79, coupled out from the prism 80and do not reach the viewer's eye. Similarly, since surfaces 79 are onlypartially transmissive, only part of the external light rays 83 passesthrough surfaces 79 and reaches the viewer's eye, while another part ofthe light rays 85 is reflected from surfaces 79, coupled out from theprism 80 and does not reach the viewer's eye, as well. Naturally, theefficiency of the projected image can be increased on account of theexternal scene, and vice-versa, namely, by increasing the reflectivityof the partially surfaces 79 the brightness of the coupled rays 63 and68 is increased. Consequently, however, the transmissivity of surfaces79 is decreased, and hence, the brightness of the external image 83 isreduced accordingly.

In contradiction to the embodiments illustrated in FIGS. 14, thecombiner 79 that reflect the coupled-out light from the substrate to theviewer's eye and at the same time transmits the external rays, is aconventional partially reflecting mirror without any special orcomplicated characteristics as surfaces 22 and 50 of the embodimentsillustrated in FIGS. 1 and 4 respectively. As a result, it is possibleto dynamically control the reflectivity (and consequently, thetransmissivity) of the partially reflective surfaces 79 according to theexternal lighting conditions and the specific image which is projectedto the viewer's eye.

One method to control the reflectivity of surfaces 79 is by using anelectrically switchable transreflective mirror, which is a solid-statethin film device made from a special liquid crystal material, and whichcan be rapidly switched between pure reflection, partial-reflection, andtotal transparent states. The required state of the switchable mirrorcan be set either manually by the user or automatically by using aphotometer which controls the reflectivity of the mirror according tothe external brightness. For the sake of simplicity, it will be assumedhenceforth that the absorption of the dynamic partially reflectingdevice is negligible, and that the sum of the reflectivity and thetransmissivity of the device is summed up to a value of approximatelyone.

FIGS. 6A and 6B illustrate use of the switchable mirror in two extremesituations. FIG. 6A illustrates a condition in which the external sceneshould be blocked from interfering with the projected image, forexample, wherein a video movie is projected, and the brightness of theexternal scene is relatively high. As shown, the dynamic surface 79 isswitched into a total-reflection state and, as a result, the coupled outlight rays 63 and 68 from the substrate are totally reflected fromsurface 79 to the viewer's eye, while the external rays 83 are totallyreflected, as well, and hence, are prevented from reaching the viewer'seye. FIG. 6B illustrates a different condition wherein it is essentialnot to block the image from the external scene at all, and it is notnecessary at that moment to project information from the coupled imageinto the viewer's eye. As shown, the dynamic surface 79 is switched intoa total-transparent state and, as a result, the coupled out light rays63 and 68 from the substrate pass substantially through surfaces 79 aand 79 b, and hence, are prevented from reaching the viewer's eye, whilethe external rays 83 pass substantially through surfaces 79 a and 79 b,as well, and hence, reach the viewer's eye undisturbed.

FIGS. 7A and 7B illustrate use of the switchable mirror in two differentintermediate situations. FIG. 7A illustrates a condition in which theprojected image should be properly combined with the external image, butthe brightness of the external scene is comparatively is high, andhence, it should be mostly blocked from interfering with the projectedimage. On the other hand, the efficiency of the projected image shouldbe high enough to achieve a reasonable contrast. As shown, the dynamicsurface 79 is switched into a primary reflection state, namely, thereflection of the switchable mirror is much higher than itstransmission. As a result, the coupled out light rays 63 and 68 from thesubstrate are mainly reflected from surface 79 to the viewer's eye,while only small part of the light waves passes through surface 79. Onthe other hand, the external rays 83 are mostly reflected from surface79 and only small part reaches the viewer's eye, FIG. 7B illustrates adifferent condition wherein the external scene is comparatively dark,and it is necessary to prevent the projected image from dazzling theviewer. As shown, the dynamic surface 79 is switched primarily into atransmission state, and thus, the reflection of the switchable mirror ismuch lower than its transmission. As a result, the coupled out lightrays 63 and 68 from the substrate mainly pass through surfaces 79 a and79 b, and hence, only a small portion of the light rays reaches theviewer's eye, while the external rays 83 mostly pass through surfaces 79a and 79 b, as well, and hence reach the viewer's eye substantiallyundisturbed.

Another approach for achieving the required dynamic partially reflectingelement is illustrated in FIGS. 8A-8C. As shown in FIG. 8A, an array ofparallel mirrors 791 a, 791 b, etc. is embedded inside the transparentplate 80 a. The minors are inclined at an angle of

$\frac{\alpha_{in}}{2}$

to the major surface 72 of the substrate. The fill-factor of the mirrorsinside the plate is substantially a half. Assuming that the projectionof a mirror on the major surface 72 is d, then the lateral distancebetween two adjacent minors is d. Another identical plate 80 b islocated adjacent to plate 80 a. As illustrated in FIG. 8A, the edges ofthe plates are located adjacent to each other, and each mirror 792 i(i=a,b,c . . . ) in plate 80 b is positioned exactly below the mirror791 i in plate 80 a. As a result, the reflectivity, and consequently,the transmissivity of the embodiment of FIG. 8A, is substantially 50%for the coupled-out image waves, as well as for the light waves from theexternal scene. As illustrated in FIG. is 8B, plate 80 b is translatedby a distance of d/2 in relation to plate 80 a, resulting in thereflection-transmission ratio of the embodiment being modified toapproximately the ratio of 75%/25%. In the embodiment of FIG. 5C, plate80 b is translated by a distance din relation to plate 80 a, and theembodiment is substantially reflective. Eventually, plate 80 b can betranslated by any other intermediate distance, and hence, thereflection-transmission ratio of the embodiment can be any value between50%:50% and 100%:0%.

The main drawback of the embodiment illustrated in FIGS. 8A-8C is thatthe maximum achievable transmissivity is limited by the value of 50%.This fault is severe for optical systems wherein the transmissivityshould be comparatively high to let the external scene reach theviewer's eye with minimal interference. FIGS. 9A-9C illustrate anembodiment composed of three identical transparent plates, wherein thefill-factor of the embedded mirrors is ⅓, namely, assuming that theprojection of a mirror on the major surface 72 is d, then the lateraldistance between two adjacent minors is 2d. As illustrated, thedistances between the edges of two adjacent plates are 0, d, and 2d, andconsequently, the reflection-transmission ratios are substantially33%:67%, 67%:33% and 100% :0% for the embodiments of FIGS. 9A, 9B, and9C, respectively. Eventually, plates 80 b and 80 c can be translated byany other intermediate distances, and hence, the reflection-transmissionratio of the embodiment can be any value between 33%:67% and 100% :0%.As a result, the systems illustrated in FIGS. 9A-9C have a higherdynamic range as compared to that of FIGS. 8A-8B and the maximalachievable transmissivity is 67% instead of 50%. The dynamic range canbe even further increased by using embodiments having larger numbers ofidentical plates. For example, for an embodiment having a plates whereinin each plate the fill-factor of the mirrors is 1/n, thereflection-transmission ratio of the embodiment can be any value between

$\frac{1}{n}\text{:}\mspace{11mu}\frac{n - 1}{n}\mspace{14mu}{and}\mspace{20mu} 100\;\%\text{:}\mspace{20mu} 0{\%.}$

Hitherto, it has been assumed that the reflectivity of the partiallyreflecting surface 79 can be modified, and hence, the ratio between theefficiencies of the virtual image coupled out from the substrate and theexternal scene can be dynamically modified to achieve optimalperformance in a given scenario. In all the embodiments illustrated thusfar, however, improving the efficiency of one of the two images is tothe detriment of the other image, that is to say, it is not possible toachieve a system wherein the efficiencies of the projected and theexternal images are very high simultaneously, using theabove-illustrated embodiments. For a dynamic partially reflectingelement, however, having a switching time which is smaller than theframe-time of the image which is projected into the substrate, it ispossible to improve the total efficiency of the system, namely it ispossible to increase the brightness of the projected image as well asthat of the external image, which reaches the viewer's eye withoutincreasing the power consumption of the optical system.

FIGS. 10A-10B illustrate optical characteristics of a conventionalsystem during a period of one frame-time τ_(f). It is assumed that theaverage brightness of the coupled-in image Bo and the external sceneB_(s) are constants and that the reflection of the partially reflectiveelement is substantially 50%, i.e., the potential efficiency is equallydivided between the projected and the external images and particularlythe brightness of the virtual image and that of the external scene whichare projected into the viewer's eye (neglecting residual losses insidethe substrate and Fresnel reflections from the external surfaces), areB₀/ 2 and B_(s)/ 2, respectively.

FIG. 11A illustrates a modified system wherein during each frame-timethe image is projected from the display source and coupled into thesubstrate only during a limited time slot having a period of t_(f)/n.The average brightness of the coupled-in image during that time slot isincreased by a factor of n to n·B₀. It is assumed that the brightness ofthe projected image depends linearly on the power consumption of thedisplay source. Since the product of the operation period with theaverage brightness is identical for the two systems of FIGS. 10A and11A, they will have substantially the same power consumption.

FIG. 11B illustrates the reflection curve of the partially reflectingelement wherein this element is substantially reflective only during alimited time slot having a period of τ_(i) f/ n, wherein this time slotis synchronized with that of the projected brightness illustrated inFIG. 11A, During the rest of the frame-time, the partially reflectiveelement is substantially transmissive. As a result, the averagebrightness of the projected image is increased by a factor of two fromB₀/2 to B₀, while the brightness of the external scene is increased to

$\frac{n - 1}{n} \cdot {B_{a}.}$

Naturally, by reducing the actual switching time, namely, by increasingthe factor n, it is possible to improve the efficiency of the externalscene.

FIGS. 12A and 12B illustrate a modified version of the system shown inFIGS. 11A and 11B. The display source here is based on a time sequentialcolor imaging, in which the color images are generated by sequentiallylaying down three basic colors of red, green, and blue (RUB) light in asingle image frame, which typically lasts of a second, where f is thefrequency of the system, usually 50 or 60 hertz. The frame-time τ_(f) isdivided into three equal, sub-periods τ_(f)/3, wherein in each one, onlyone color is illuminating the display. It is also assumed that thedynamic partially reflecting element can be controlled to yield a highreflection in each one of the primary three colors while having at thesame time high transmittance for the other two colors. As illustrated inFIG. 12A, during each frame-time for each of the three primary colors,the image is projected from the display source and coupled into thesubstrate only during a limited time slot having a period of τ_(f)/n.The average brightness of the coupled-in image during that time slot is

$\frac{n \cdot B_{0}}{3}.$

FIG. 12B illustrates the reflection curve of the partially reflectingelement wherein this element is substantially reflective for each of thethree primary colors only during a limited time slot having a period often, wherein each of these three slots is synchronized with therespective slot of the projected brightness as illustrated in FIG. 12A.The average brightness of the projected image is B₀, while thebrightness of the external scene is

$\frac{n - 1}{n} \cdot {B_{s}.}$

The exploitation of a dynamic partially reflecting element can beparticularly advantageous for the multi-facet element 255 illustrated inFIG. 13A. As shown, a reflecting surface 256 is embedded inside thesubstrate 258. Surface 256 has the same reflecting characteristics assurface 67 and is parallel to the coupling-in and the coupling-outsurfaces 65 and 67. A ray 260 having an off-axis angle of α_(in) iscoupled into the substrate 258 after one reflection from surface 65, andafter a few reflections from the major surfaces of the substrate 258impinges on surface 256. The ray is coupled out from the substrate 258and is then partially reflected into the viewer's eye in a similarmanner as to that which is illustrated in FIGS. 5A-5B. The reflected rayis, however, in this case is not propagated undisturbed into theviewer's eye, as in the embodiments illustrated in FIGS. 5A and 5B.Instead, the reflected ray impinges on a partially reflecting surface264 a, which is parallel to surface 79 a and coupled inside a flat prism267, which is attached to the upper surface 70 of the substrate 268.Part of the intensity of the light ray 260 which impinges on surface 264a, passes through the surface as ray 260 a and continues to propagatetoward the viewer's eye. Since surfaces 79 a and 264 a are parallel, theother part of the intensity of the light ray 260 is reflected fromsurface 264 a as ray 260 b having an off-axis angle of α_(in) andimpinges again on surface 256. After two reflections from surface 256,it propagates inside the substrate 258, and after two more reflectionsfrom the coupling-out surface 67, the ray 260 b is coupled out fromsubstrate 258 having the same off-axis angle α_(in). The ray is thenreflected from surface 79 d, which is parallel to surface 79 a, into theviewer's eye having the same direction as ray 260 a.

As also illustrated in FIG. 13A, another ray 262 is coupled into thesubstrate 258 after two reflections from surface 65, and after a fewreflections from the major surfaces of the substrate 258, the ray 262impinges on surface 256. The ray is coupled out from the substrate 258having an off-axis angle α_(in) and is then partially reflected bysurface 79 b, which is parallel to surface 79 a, into the viewer's eyein a similar manner to ray 260. The reflected ray impinges on thepartially reflecting surface 264 b which is parallel to surfaces 79 band 264 a and is coupled inside prism 267. Part of the intensity of thelight ray 262, which impinges on surface 264 b, passes through thesurface as ray 262 a and continues to propagate toward the viewer's eye.Since surfaces 796 and 264 b are parallel, the other part of theintensity of the light ray 262 is reflected from surface 264 b as ray262 b having an off-axis angle of α_(in), and impinges again on surface256. After one reflection from surface 256 it propagates inside thesubstrate 258, and after one reflection from the coupling-out surface67, the ray 262 b is coupled out from substrate 258 having the sameoff-axis angle α_(in). The ray is then reflected from surface 79 c,which is parallel to surface 79 b, into the viewer's eye having the samedirection as ray 260 a. Hence, all four of the rays, 260 a, 260 b, 262a, and 262 b, which originated from the same point on the displaysource, reach the viewer's eye having the same propagating direction.

As a result, the output aperture of substrate 258 is the combination ofsurfaces 256 and 67. Consequently, the active area of the outputaperture of substrate 258 has been doubled as compared to that ofsubstrate 64, which is illustrated in FIG. 5, while the thickness of thesubstrate remains the same. On the other hand, the brightness of lightwaves coupled out from substrate 258 has been reduced by a factor of twoas compared to that of substrate 64. Furthermore, not only thecoupled-out light waves 260 and 262 of the virtual image impinge on thepartially reflecting surfaces 264A and 264B, but also rays 82 from theexternal scene. As a result, the brightness of these waves is reduced bythe same factor accordingly. There are ways, however, to improve thebrightness of the coupled-out light waves of both images, Forembodiments wherein the light waves coupled inside the substrate arelinearly polarized, such as systems where the display source is an LCDor an LCOS display, the partially reflecting surfaces 79 i, as well as264 i (i=a, b, . . . ), can be designed to be polarization-sensitivereflecting surfaces. These surfaces are reflective (or partiallyreflective) for one polarization (preferably for the s-polarization) andsubstantially transparent to the orthogonal polarization (preferably forthe p-polarization). In such a case, the transmittance of the externalscene for see-through applications can be improved, since the entireelement 255 is now substantially transparent to the polarization (whichis orthogonal to that of the light waves coupled inside the substrate).While the reflecting surfaces 79 i can be totally reflective for therelevant polarization (which is the same as that of the light wavescoupled inside the substrate), surfaces 264 i should be partiallyreflective for this polarization, wherein the exact reflectioncoefficient of the surfaces can be determined according to the number ofreflecting surfaces 264 i in the system. For the embodiment illustratedin FIG. 13A, wherein two reflective surfaces 256 and 67 are embeddedinside the substrate 258, a reflection coefficient of 0.5 can yield atotal brightness efficiency of 50% for the light waves coupled insidethe substrate and transmittance of 50% for the external scene.

An alternative embodiment for improving the efficiencies of both of theimages, which can be applied to polarized as well as non-polarized imagesources, is illustrated in FIGS. 13B and 13C. Here, surfaces 79 i, inaddition to surfaces 264 i, are dynamic reflecting surfaces. Asillustrated in FIGS. 11A and 11B, during each frame-time the image isprojected from the display source and coupled into the substrate onlyduring a limited time slot having a period of τ_(f)i/n wherein theaverage brightness of the coupled-in image is n·B₀. In addition, asillustrated in FIG. 13B, the elements 79 i and 264 i are reflective onlyduring the same time slot, wherein surfaces 79 i are substantiallyreflective and the reflectivity of surfaces 264 i is around 50%. Asillustrated in FIG. 13C, during the rest of the frame-time, thereflective elements 79 i and 264 i are substantially transmissive, andthe external light rays reach the viewer's eye without any interference.As a result, the average brightness of the projected image even for anon-polarized image is increased by a factor of two from B₀/4 to B₀/2,while the brightness of the external scene is increased to

$\frac{n - 1}{n} \cdot {B_{s}.}$

The embodiment for expanding the output aperture by embedding areflecting surface 256 into the substrate 258, as illustrated in FIGS.13A and 13B, is not limited to a single reflecting surface. For opticalsystems having wide FOVs together with relatively large EMB values, anarray of n flat reflecting surfaces 256 i (i=a,b . . . ), which areparallel to the output reflecting surface 67, can be embedded internallyinside the substrate to increase the output aperture of the substrate bya factor of Consequently, the number of the reflecting surfaces 264 i(i=a,b . . . ) should be increased accordingly, to completely cover theoutput aperture of the embedded surfaces 256 i. The reflectance andlateral extension of each reflecting surface 264 i should be designed toensure the uniformity is characteristics of the light waves coupled intothe viewer's eye.

As illustrated in FIGS. 14A to 14D, the efficiency of a system having anadditional number of coupling-out facets 256 i can be increased byadding an eyeball tracking unit 272 to the optical system. Eyeballtracking is the process of measuring either the location, the point ofgaze, or the motion of an eye relative to the display, namely, aneyeball tracker is a device for measuring eye positions and eyemovement. The most popular method for operating this device is byutilizing an optical method for measuring eye motion. Light from anemitter, typically infrared, is reflected from the eye and sensed by avideo camera, or some other specially designed optical sensors. Theinformation is then analyzed to extract eye rotation and translationfrom changes in reflections. Video-based eye trackers typically usecorneal reflection and the center of the pupil as features to track overtime.

In accordance with the present invention, it would be advantageous tophysically combine the two optical units; namely, the dynamicallycontrolled stereoscopic display and the eyeball tracking unit. Thesystem should also contain a dynamic control unit 276, which will becapable of setting, by identifying the position and gazing point of theviewer's eyes, the operation times and the reflectance for each of thereflecting surfaces 264 i. Seen in FIG. 14A is an optical system,wherein three coupling out surfaces, 256 a, 256 b and 67 are embeddedinside the substrate 258. To achieve coupled-out light waves having auniform brightness over the entire output aperture, the reflectance ofthe partially surfaces 264 i, which are adjacent to the coupling-outsurfaces 256 a and 256 b, should be set to 67% and 50%, respectively.The maximal efficiency of the coupled-out light waves, in that case, isapproximately 33%.

For optical systems, however, having a large EMB values, not all thelight waves which are coupled out from the substrate are exploitedsimultaneously. As a result, the efficiency of the optical system can beimproved by increasing the brightness of the light waves that reach theviewer's eye on account of the other light waves. As illustrated in FIG.14B, the viewer's eye is located in the right section of the EMB,Consequently, the eye is illuminated mostly by light waves which arecoupled out by surface 256 a, and hence, it would be advantageous toincrease the brightness of these light waves on account of those whichare coupled out by surfaces 256 b and 67. This improvement can beachieved by decreasing the reflectance of surfaces 264 a and 264 b,which are adjacent to surface 256 a. Therefore, most of the light waveswill pass through surfaces 264 a and 264 b into the viewer's eye, andonly a small part thereof will be reflected and coupled-in again intothe substrate.

A different situation is illustrated in FIG. 14C, wherein the viewer'seye is located in the central section of the EMB. Consequently, the eyeis illuminated mostly by light waves which are coupled out by surface256 b, and hence, it would be advantageous to increase the brightness ofthese light waves on account of those which are coupled out by surfaces256 a and 67. This improvement can be achieved by increasing thereflectance of surfaces 264 a and 264 b, which are adjacent to surface256 a, and reducing the reflectance of surfaces 264 c and 264 d, whichare adjacent to surface 256 b. Therefore, most of the light waves willbe reflected from surfaces 264 a and 264 b, coupled-in again intosubstrate 258 and coupled out by surface 256 b. Now, most of theremaining light waves will pass through surfaces 264 c and 264 d intothe viewer's eye, and only a small part thereof will be reflected andcoupled-in again into the substrate.

A further different situation is illustrated in FIG. 14D, wherein theviewer's eye is located in the left section of the EMB. Consequently,the eye is illuminated mostly by light waves which are coupled out bysurface 67, and hence, it would be advantageous to increase thebrightness of these light waves on account of those which are coupledout by surfaces 256 a and 256 b. This improvement can be achieved byincreasing the reflectance of surfaces 264 a and 264 b, as well assurfaces 264 c and 264 d, which are adjacent to surfaces 256 a and 256b, respectively. As a result, most of the light waves will be reflectedfrom surfaces 264 a and 264 b, as well as from surfaces 264 c and 264 d,will be coupled-in again into substrate 258, and then coupled out bysurface 67 to reach the viewer's eye s undisturbed. Eventually, theexact values of the reflectance of surfaces 264 i will be set by thecontrol unit according to the position and gazing point of the viewer'seyes, the EMB and the FOV of the optical system, and other possiblerelevant parameters.

Hereinbefore, it has been assumed that the reflectivity (and thereforethe transmissivity) of the dynamic partially reflective element isconstant over its entire aperture. There are situations, however, whereit would be beneficial to use a dynamic element wherein its reflectancecan be modified locally. That is to say, different parts of the dynamicelement will have various degrees of reflectivity. By combining thisability with the eyeball tracking unit 272 and the dynamic control unit276, it will be possible to adjust the local reflection of the dynamicelement optimally. The exact localized reflectivity can be set accordingto the type of the information which is projected to the viewers eye,the location of the symbols or the video in the FOV of the image, thebrightness of the external. scene, and the position and the gazing pointof the viewer's eyes.

FIGS. 15A and 15B illustrate possible embodiments for achieving arequested dynamic element. As shown in FIG. 15A, the active area ofreflectivity the electrically switchable transreflective mirror 280 ispixelized into a two-dimensional array of pixels 281, wherein thereflectivity of each pixel can be separately set by the dynamic controlunit 276 (not shown). As demonstrated in FIG. 15B, a sub-area 282 ofelement 280 has been set to be substantially reflective; another twosub-areas 283 and 286 are partially reflective while the other activearea 289 of element 280 is substantially transmissive. A differentapproach for achieving the required dynamic partially reflective elementis illustrated in FIGS. 16A and 16B. As shown in FIG. 16A, the dynamicelement is composed of two identical arrays (291 and 292) of sub-mirrors295, wherein each sub-mirror can be separately translated by, thedynamic control unit 276 (not shown). Since the sub-mirror can be smalland light elements, it is possible to translate them using piezoelectricdevices. The fill factor of each array is 50%, and each sub-mirror inarray 291 is located substantially adjacent to the relative sub-minor inarray 292. As a result, the reflectivity and the transmittance ofelement 290 here are around 50% over the entire area) of the dynamicelement 290. FIG. 16B illustrates a different situation wherein in twodifferent locations 296 and 297 some sub-mirrors in array 292 have beenlaterally translated to yield high reflectance in these areas.

In the embodiment illustrated in FIGS. 13A to 13C and 14A to 14D, theoutput aperture of the optical system has been extended utilizingexternal partially reflecting surfaces 264, which were attached to theupper surface 70 of the substrate 258. FIGS. 17A to 17C illustrate analternative embodiment for expanding the output aperture wherein thecoupling-out surface 256 is sensitive to the incident angle of thecoupled light waves. As shown in FIG. 17A, optical rays 320 a and 320 bhaving an input direction of α_(in) ⁽⁰⁾ impinge on an optical element309, composed of two substrates 310 a and 310 b, wherein the lowersurface 311 a of substrate 310 a is attached to the upper surface 312 bof substrate 310 b defining an interface plane 317.

There are two contradicting requirements from the interface plane 317between the substrates 310 a and 310 b. On the one hand, the first twoorders image F⁽¹⁾ and F⁽²⁾ should be reflected from that plane, whilethe zero order image FM from the upper substrate 310 a shouldsubstantially pass through it, after being reflected from surfaces 256and 67, with no significant reflections. Similarly, surface 317 shouldbe transparent to rays 320 a and 320 b entering the substrate throughelement 318 having the input angle of α_(in) ⁽⁰⁾. In addition, forsee-through systems the transparency of the optical system forsubstantially normal incident light, should be as high as possible. Apossible way to achieve this is to use an air gap in the interface plane317, however, for achieving a rigid system, it is preferable to apply anoptical adhesive in the interface plane 317, in order to cement thesubstrates 310 a and 310 b. This approach is illustrated with an opticalsystem having the following parameters:

α_(sur1)=α_(sur2)=9°; F ⁽⁰⁾={32°, 44°}; F ⁽¹⁾={50°, 62°}

F ⁽²⁾={68°, 80°}.  (8)

The light waves are s-polarized. The optical material of the substrates310 a and 310 b is Schott N-SF57 having a refractive index ofν_(d)=1.8467, and the optical adhesive is NOA 1315, having a refractiveindex of ν_(d)=1.315. The critical angle is therefore α_(cr)>45.4°. Allthe optical rays in the higher orders F⁽¹⁾ and F⁽²⁾ have off-axis angleshigher than the critical angle and are therefore totally reflected fromthe interface plane 317. All the optical. rays in the zero order impingeon the interface plane at an incident angle lower than the criticalangle, and hence, they pass through it. To minimize the Fresnelreflections of the coupled-out light waves from the interface plane,however, it is preferred to apply a suitable anti-reflective (AR)coating to this plane.

In contradistinction to the substrates illustrated in FIGS. 13A to 14D,the coupling-out elements 256 a and 256 b are not conventionalreflecting surfaces as surface 256 in substrate 258, but angularsensitive reflective surfaces. Each one of the surfaces is substantiallytotal reflective for the part of the angular range of the coupled-inlight waves inside the respective substrate, and substantiallytransparent for the other part of the angular range. Unlike surfaces 256a and 256 b, surfaces 67 a and 67 b could be conventional reflectingsurface, such as surface 67 in the embodiments of FIGS. 5A and 5B. As aresult, the efficiency of the optical system is significantly improved,and the brightness of the coupled-out image is substantially retained,similar to that of the input image. To achieve is this improvement, thefact that the light waves coupled out from the substrate are notrequired to illuminate the entire active area of the coupling-outsurface is utilized.

As illustrated in FIG. 18, showing the rays that should impinge on thepartially reflective surface 79 for illuminating the EMB 197, the twomarginal (107R, 107L) and the central (107M) light waves of the imageare coupled out from the substrate and redirected into the viewer's eye24. As shown, the light waves 107R, 107M, and 107L, having the zeroorder off-axis angles α_(in) ⁽⁰⁾(max), α_(in) ⁽⁰⁾(mid) and α_(in)⁽⁰⁾(min), illuminate only the parts 79R, 79M and 79L of the partiallyreflecting surface 79, respectively, which reflect into to EMB 197. As aresult, a method can be found where the coupled-in light waves are splitin such a way that they will illuminate only the required respectivepart of surface 79, and the original brightness will be preserved. Toachieve this, the angular range of the light waves F_(sur1)⁽¹⁾≡{α_(min), α_(max)}, which impinge on the angular sensitivecoupling-out surfaces 256 a and 256 b at the incident angles of α_(sur)⁽¹⁾=α_(in) ⁽⁰⁾+α_(sur1)=α_(in) ⁽¹⁾−α_(sur1) (FIG. 17A), is divided intothree substantially equal segments: F_(low) ⁽¹⁾≡{α_(min), α_(m1)},F_(mid) ⁽¹⁾≡{α_(m1), α_(m2)} and F_(max) ⁽¹⁾≡{α_(m2), α_(max)}. The aimof the embodiment is that the light waves having the higher incidentangles in the FOV of F_(max) ⁽¹⁾≡{α_(m2), α_(max)} will be coupled outfrom the substrates 310 a, 310 b by the angular sensitive coupling-outelement 256 a and 256 b; the light waves having the lower incidentangles in the FOV of F_(min) ⁽¹⁾≡{α_(min), α_(m1)} will be coupled outfrom the substrates 310 a, 310 b by the coupling-out element 67 a and 67b, and the light waves in the FOV of F_(mid) ⁽¹⁾≡{α_(m1), α_(m2)} willbe coupled out from the upper substrate 310 a, by the coupling-outelement 67 a and from the lower substrate 310 b, by the angularsensitive coupling-out element 256 b.

In order to achieve this, surfaces 256 a and 256 b should substantiallyreflect all the light waves in F_(max) ⁽¹⁾ such that they will becoupled-out from the substrates 310 a and 310 b and substantiallytransmit all the light waves in F_(min) ⁽¹⁾, such that they willcontinue to propagate inside the substrate and be coupled-out by thereflecting surfaces 67 a and 67 b. In addition, the light waves inF_(mid) ⁽¹⁾ should pass-through the angular sensitive surface 256 a,continue to propagate inside the substrate 310 a and be coupled-out bythe surface 67 a, but will be coupled-out from substrate 310 b by theangular sensitive surface 256 b.

Consequently, the angular sensitive reflecting surfaces 256 a and 256 bshould fulfill the following three characteristics for the entirerelevant photopic range:

-   -   a. substantially total reflective for the angular range of        {α_(m2), α_(max)};    -   b. substantially transparent for the angular range of {α_(min),        α_(m1)}; and    -   c. while the lower surface 256 b is substantially total        reflective for the angular range of {α_(m1), α_(m2)} the upper        surface 256 a is substantially transparent for the) same angular        range of {α_(m1), α_(m2)}.

It is possible to achieve these requirements by applying angularsensitive dielectric coatings on surfaces 256 a and 256 b, but theprocess for achieving these coatings can be fairly complicated. Asimpler way is to cement the optical part adjacent to surfaces 256 a and256 b using optical adhesives having proper refractive indices thatyield critical angles of α_(m1) and, α_(m2) at surfaces 256 a and 256 b,respectively. The high transparency for angles lower than the respectivecritical angles can be achieved using proper AR coatings. To simplifythe fabrication process of the angular sensitive surfaces, it is usuallyrequired that the Abbe numbers of the optical adhesive and the opticalmaterial of the substrate will be similar to avoid undesired chromaticeffects in the image. It is possible, however, to achieve the requiredreflecting curves utilizing proper thin-film coating design techniques,even for cases where the Abbe numbers of the adhesive and the opticalmaterial are substantially different.

FIG. 17A illustrates two rays 320 a and 320 b from the same plane inputwave having incident angles of α_(si) ⁽¹⁾<α_(m1) which impinge on theangular sensitive coupling-out elements 256 a and 256 b, respectively.As a result of condition (b) stated hereinabove, both rays pass throughsurfaces 256 a and 256 b. Ray 320 a is reflected three times from thecoupling-in element 65 a, trapped inside the substrate 310 a at anoff-axis angle of α_(in) ⁽³⁾=α_(in) ⁽⁰⁾+6·α_(sur1), and is reflectedtwice from surface 256 a before being impinged on the left part ofsurface 256 a at an incident angle of α_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1).Ray 320 b is reflected twice from the coupling-in element 65 b, trappedinside the substrate 310 b at an off-axis angle of α_(in) ⁽²⁾=α_(in)⁽⁰⁾+4·α_(sur1), and is reflected once from surface 256 b before beingimpinged on the left part of surface 256 b at an incident angle ofα_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). After passing through surface 256 a, ray320 a continues to propagate inside substrate 310 a at an off-axis angleof α_(in) ⁽¹⁾, after a single reflection from surface 67 a iscoupled-out from substrate 310 a. and redirected into the viewer's eyeby the partially reflecting surface 79 b. After passing through surface256 b, ray 320 b is reflected once from the left side of surface 256 b,continues to propagate inside substrate 310 b at an off-axis angle ofα_(in) ⁽²⁾, and after a double reflection from surface 67 b iscoupled-out from substrate 310 b, and redirected into the viewer's eyeby the partially reflecting surface 79 b.

FIG. 17B illustrates two rays 321 a and 321 b from the same plane inputwave having incident angles of α_(si) ⁽⁰⁾>α_(m2) which impinge on theangular sensitive coupling-out elements 256 a and 256 b, respectively.As a result of condition (a) stated hereinabove, both rays are reflectedfrom surfaces 256 a and 256 b and are coupled-out from the substrates310 a and 310 b by these angular sensitive reflective surfacesrespectively. Ray 321 a is reflected once from the coupling-in element65 a, trapped inside the substrate 310 a at an off-axis angle of α_(in)⁽¹⁾=α_(in) ⁽⁰⁾+2·α_(sur1), and impinges on the right part of surface 256a at an incident angle of α_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). Ray 320 b isreflected twice from the coupling-in element 65 b, trapped inside thesubstrate 310 b at an off-axis angle of α_(in) ⁽²⁾=α_(in)⁽⁰⁾+4·α_(sur1), and is reflected once from surface 256 b before beingimpinged on the left part of surface 256 b at an incident angle ofα_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). After being reflected and coupled-outfrom the substrates, rays 321 a and 321 b are redirected by thepartially reflecting surface 79 a into the viewer's eye. FIG. 17Cillustrates two rays 322 a and 322 b from the same plane input wavehaving incident angles of α_(m1)<α_(si) ⁽⁰⁾<α_(m2,) which impinge on theangular sensitive coupling-out elements 256 a and 256 b, respectively.Ray 322 a is reflected once from the coupling-in element 65 a, trappedinside the substrate 310 a at an off-axis angle of α_(in) ⁽¹⁾=α_(in)⁽⁰⁾+2·α_(sur1), and impinges on the right part of surface 256 a at anincident angle of α_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). As a result ofcondition (c) stated hereinabove, ray 322 a passes through surfaces 256a and after a single reflection from the left side of surface 256 a, ray322 a continues to propagate inside substrate 310 a at an off-axis angleof α_(in) ⁽²⁾, and after a double reflection from surface 67 a iscoupled-out from substrate 310 a, and redirected into the viewer's eyeby the partially reflecting surface 79 b. Ray 322 b is reflected oncefrom the coupling-in element 65 b, trapped inside the substrate 310 b atan off-axis angle of α_(in) ⁽¹⁾=α_(in) ⁽⁰⁾+2·α_(sur1), and impinges onthe right part of surface 256 b at an incident angle of α_(in)⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). As a result of condition (c) statedhereinabove, ray 322 b is reflected from surface 256 b, coupled-out fromthe substrate 310 b, and is redirected by the partially reflectingsurface 79 a into the viewer's eye.

The implementation of the angular sensitive reflecting surfaces 256 aand 256 b utilized in the embodiments of FIGS. 17A, 17B and 17C isillustrated herein with an optical system having the followingparameters for the optical system 309:

α_(sur1)=α_(sur2); =9°; F ⁽⁰⁾={32°, 44°}; F ⁽¹⁾={50°, 62°}

F ⁽²⁾={68°, 80°}; F _(sur1) ⁽¹⁾={41°, 53°}; F _(sur1) ⁽²⁾={59°,71°},  (9)

The light waves are s-polarized. The optical material of the substrate64 using Schott N-SF57 having a refractive index of ν_(d)=1.846, and theoptical adhesives which are adjacent to surfaces 256 a and 256 b inFIGS. 17A-17C are NOA-139 and NOA 1315, having refractive indices ofν_(d)=1.315 and ν_(d)=1.39, respectively. The overall FOV of thecoupled-in image is F⁽⁰⁾={32°, 44°} (which is practically a FOV of 22°in the air) and the angular range of F_(sur1) ⁽⁰⁾={41°, 53°} is dividedinto three substantially equal segments: F_(low) ⁰={41°, 45°}, F_(mid)⁽⁰⁾={45°, 49°} and F_(max) ⁽⁰⁾{=49°, 53°}.

FIG. 19A illustrates the graph of the reflection from the angularsensitive reflective surface 256 b in FIGS. 17A, 17B and 17C coated withan appropriate AR dielectric coating as a function of the incident anglefor three different wavelengths: 450 nm, 550 nm and 650 nm. As shown,the reflection is 100%, due to total internal reflection for angularspectrum above 45.4°, while it is very low for the incident angles of{41°, 44.9°}. FIG. 19B illustrates the graph of the reflection from theangular sensitive reflective surface 256 a in FIGS. 17A, 17B and 17Ccoated with an appropriate AR dielectric coating as a function of theincident angle for three different wavelengths: 450 nm, 550 nm and 650nm. As shown, the reflection is 100%, due to total internal reflection,for angular spectrum above 48.8°, while it is very low for the incidentangles of {41°, 48.6°}.

Since each one of the two substrates 310 a, 310 b functionsindependently, there are no longer any constraints on the co-linearityof each adjacent coupling-in and coupling-out surfaces. The onlyconstraint is that for each separate substrate 310 a or 310 b, the majorsurfaces and the coupling-in and the coupling-out surfaces should beparallel to each other, respectively. Moreover, each separate substratecan have a different thickness and a different inclination angle,according to the requirements of the optical system.

FIG. 20 illustrates the two marginal and the central light waves of theimage which are coupled out from the substrate and redirected into theviewer's eye 24. As shown, the light waves 320, 321 and 322, having thezero order off-axis angles of α_(in) ⁽⁰⁾(min), α_(in) ⁽⁰⁾(max) andα_(in) ⁽⁰⁾(mid), are illuminating each only the parts of the partiallyreflecting surfaces 79 a and 79 b which are required to illuminate theEMB 197; the rightmost light wave 321 (dashed-dotted lines) is reflectedonly from the right surface 79 a; the leftmost light wave 320 (dottedlines) is reflected only from the left surface 79 b, while the centrallight wave 322 (dashed lines) is reflected from the left part of theright surface 79 a, and the right part of the left surface 79 b. All thelight waves reach the viewers eye over the entire extent of the EMB 197.Consequently, the EMB 197 of the embodiment illustrated in FIGS. 17A to17C and 20 has the same brightness as the embodiment illustrated inFIGS. 5A and 5B, while the output aperture is doubled. Apparently, thebrightness of the coupled-out light waves can be increased furthermoreby exploiting partially reflecting surfaces 79, which can be dynamicallycontrolled according to the methods illustrated beforehand in relationto FIGS. 6A to 12B. As a result, the brightness of the coupled-out imagewaves which reaches the viewer's eye can be increased to a level veryclose to the brightness of the light waves coupled into the substrate,namely, the brightness efficiency of the element can be nearly 1.

FIGS. 17A to 17C and 20 illustrate outlines of embodiments comprising apair of substrates, and two angular sensitive coupling-out surfacesembedded respectively inside s these substrates, wherein the outputaperture is increased by a factor of two without reducing the brightnessof the projected image. There are systems, however, having a wide FOYand an input aperture remotely located from the EMB, which significantlyincrease the required output aperture of the main substrate. In thesecases, increasing the aperture by a factor of two in not enough and ahigher increasing factor is required. To achieve this goal, theabove-illustrated increasing method can be generalized to increasingfactors of n>2.

Assuming that it is necessary to increase the aperture of the image by afactor of n, as illustrated in FIG. 21, n−1 pairs of angular sensitivecoupling-out surfaces should be inserted respectively into thesubstrates. For each pair, the surfaces should be adjacently located inthe same manner as surfaces 256 a and 256 b are located in substrates310 a and 310 b. In addition, the projection of the lower angularsensitive coupling-out surface, over the major surface of the substrate,should be adjacently located to the projection of the upper angularsensitive coupling-out surface of the consecutive pair. The angularrange of the light waves which impinge on the coupling-out surfacesF_(sur1)≡{α_(min), α_(max)} is divided now into 2n−1 substantially equalsegments, by setting 2n−2 equally separated angles α_(j). That is,F₁≡{α_(min), α₁}, F₂≡{α₁, α₂} . . . F_(j)≡{α_(j−1), α_(j)} andF_(2n−1)≡{α_(2n−2), α_(max)}. Assuming that the angular sensitivecoupling-out surfaces are denoted as S_(j), where j is the running indexfrom right (j=1) to left (j=2n−2), then each surface S_(j) of the 2n−2elements should fulfill the following conditions for the entire relevantphotopic range:

-   -   a. substantially totally reflective for the angular range of        α_(si) ⁽⁰⁾>α_(2n−j−1), and    -   b. substantially transparent for the angular range of α_(si)        ⁽⁰⁾<α_(2n−j−1),

wherein, the coupling-out element S_(j) should reflect all the impinginglight waves having incident angles higher than the limit angle ofα_(2n−j−1), to couple-out these light waves from the substrate, and tosubstantially transmit all the other light waves toward the nextcoupling-out element S_(j+2). As explained above, the simplest way toachieve these requirements is to cement the optical parts adjacent tothe respective coupling-out surface, using optical adhesives havingproper refractive indices that yield critical angles of α_(2n−j−1).Also, as previously described, the high transparency for incident angleslower than the respective critical angles, can be achieved using properAR coatings.

The above illustrated embodiments, comprising n−I pairs of angularsensitive coupling-out surfaces, will have the followingcharacteristics:

-   -   a. The light waves which are coupled-out by each surface S_(j)        (j=1 . . . 2n−2) are those in the angular range of {α_(2n−j−1),        α_(2n−j+1)} (α_(min) and α_(max) are denoted here as α₀ and a²⁻¹        respectively). The light waves coupled-out by the conventional        coupling-out element 67 a and 67 b are those in the angular        ranges of {α₀, α₂} and {α₀, α₁}, all, respectively, while the        light waves which are coupled-out by the first surface S_(l) are        those in the angular range of {α_(2n−2), α_(max)}.    -   b. each light wave (inside the angular range of the light waves        which impinge on the input surface of the upper pair        F_(sur1)≡{α_(min), α_(max)})having an incident angle of        α_(j−1)<α_(s)<α_(j) (j=1 . . . 2n−1), is coupled-out by two        adjacent surfaces—S_(2n−j) and S_(2n−+1) and is consequently        redirected into the viewer's eye by the respective part of the k        partially reflecting surfaces 79 _(i) (i=I . . . k). Therefore,        each light wave which is coupled inside the embodiment by total        internal reflection, is coupled out by 1/n part of the overall        coupling-out element. By proper design, however, substantially        all the coupled light waves will cover the designated EMB of the        system.

It has been previously assumed that two adjacent substrates areexploited to increase the output aperture by a factor of n withoutreducing the brightness of the projected image. For systems having arelatively wide FOV, however, it will be more appropriate to utilizethree, instead of two adjacent substrates. In that case, as illustratedin FIG. 22, n−1 triplets of angular sensitive coupling-out surfacesshould be inserted respectively into the three substrates, 301 a, 310 b,and 310 c. For each triplet the surfaces should be adjacently located,and the projection of the lower angular sensitive coupling-out surfaceshould be adjacently located to the projection of the upper angularsensitive coupling-out surface of the consecutive triplet, in the samemanner described heretofore in relation to the double substrate. Theangular range of the light waves which impinge on the coupling-outsurfaces F_(sur1)≡{α_(min), α_(max)} is divided now into 3n−2substantially equal segments, by setting 3n−3 equally separated anglesα_(j). That is, F₁≡{α_(min), α₁}, F₂≡{α₁, α₂} . . . F_(j)≡{α_(j−1),α_(j)} and F_(3n−2)≡{α_(3n−3), α_(max)}. As previously, each surfaceS_(j) of the 3n−3 elements should be substantially totally reflectivefor the angular range of α_(si) ⁽⁰⁾>α_(3n−j−2), and substantiallytransparent for the angular range of α_(si) ⁽⁰⁾<α_(3n−j−2).

That is to say, the coupling-out element S_(j) should reflect all theimpinging light waves having incident angles higher than the limit angleof α_(3n−j−2), to couple-out these light waves from the substrate, andto substantially transmit all the other light waves toward the nextcoupling-out element S_(j+3).

The above illustrated embodiments, comprising n−1 triplets of angularsensitive to coupling-out surfaces, will have the followingcharacteristics:

a. The light waves which are coupled-out by each surface S_(j)(j =1 . .. 3n−3) are those in the angular range of {α_(3n−j−2), α_(2n−j+1)}(α_(min) and α_(max) are denoted here as α₀ and α_(3n−2) respectively).The light waves coupled-out by the conventional coupling-out element 67a, 67 b and 67 c are those in the angular ranges of {α₀, α₃}, {α₀, α₂}and {α₀, α₁}, respectively, while the light waves which are coupled-outby the first surface S_(j) are those in the angular range of {α_(3n−n),α_(max)}.

-   -   b. each light wave (inside the annular range of        F_(sur1)≡{α_(min), α_(max)}) having an incident angle of        α_(j−1)<α_(s)<α_(j) (j=1 . . . 3n−2), is coupled-out by three        adjacent substrates—S_(3n−j−1), S_(3n−j) and S_(3n−j+1) and is        consequently redirected into the viewer's eye by the respective        part of the k partially reflecting surfaces 79 _(i) (i=l . . .        k).

Clearly, the number of the adjacent substrates that can be exploited toincrease to output aperture is not limited to three. Any number m ofadjacent substrates and (n−1)·m of angular sensitive surfaces can beutilized according to the various parameters of the optical system.

FIG. 23 illustrates an alternative embodiment for expanding the outputaperture wherein not only the coupling-out surfaces, but also some ofthe coupling-in surfaces, are sensitive to the incident angle of theinput waves. As shown, an optical ray 364 impinges on an optical element355, composed of two substrates 360 a and 360 b, wherein the lowersurface 361 b of substrate 360 a is attached to the upper surface 361 cof substrate 360 b defining an interface plane 368. The coupling-inelement 365 of the first substrate 360 a is an angular sensitivereflecting surface, wherein the coupling-in element 366 of the lowersurface is a reflecting surface which is located beneath the surface365. The input ray 364, which enters the upper substrate 360 a throughthe front surface 363 of the intermediate prism 367, can either betotally reflected by surface 365 and coupled inside the upper substrate360 a (dashed line), can substantially pass through surface 364 to becoupled by surface 367 inside the lower substrate 360 b (dotted line),or may be partially reflected by surface 364 and coupled insidesubstrate 360 a, as well as inside substrate 360 b.

Ray 364 a, which is coupled inside the upper substrate 360 a, can eitherbe totally reflected by the angular sensitive reflecting surface 362 ato be coupled out from element 355 as ray 364 aa, or substantiallypass-through surface 362 a to be coupled again by surface 362 a insidethe upper substrate 360 a and coupled out from the element 355 by thecoupling-out element 67 a as ray 364 ab, or may be partially reflectedby surface 362 a and coupled out from element as rays 364 aa and 364 ab.

Ray 364 b, which is coupled inside the lower substrate 360 b, can eitherbe totally reflected by the angular sensitive reflecting surface 362 bto be coupled out from element 355 as ray 364 ba, or could substantiallypass-through surface 362 b to be coupled again by surface 362 b insidethe lower substrate 360 b and be coupled-out from the element 355 by thecoupling-out element 67 b as ray 364 bb, or be partially reflected bysurface 362 b and be coupled out from element as rays 364 ba and rays364 bb.

The simplest way to obtain element 355 is by designing elements 362 a,362 b and 365 as conventional beamsplitters which are not sensitive tothe incident angle of the input waves. As a result, each input ray willbe evenly split by the partially reflecting surfaces, and hence, becoupled-out from all the reflecting surfaces as rays 364 aa, 364 ab, 364ba and 364 bb. Consequently, for each input light wave the outputaperture will be the projection of surfaces 362 a, 362 b, 67 a and 67 bon the lower surface 361 d. The output aperture is expanded by a factorof 4 compared to the input aperture, which is the projection of surfaces365 and 367 on surface 361 d. Accordingly, however, the brightness ofthe coupled-out light wave is attenuated by a factor of 4 as compared tothat of the input light wave.

Another way, to achieve element 355 is to divide the angular range ofthe light waves F_(sur1) ⁽¹⁾≡{α_(min), α_(max)}, which impinge on theangular sensitive coupling-in surface 365 and the coupling-out surfaces362 a and 362 b at the incident angles of α_(sur) ⁽¹⁾=α_(in)⁽⁰⁾+α_(sur1)=α_(in) ⁽¹⁾−α_(sur1), into four substantially equalsegments: F_(low) ⁽¹⁾≡{α_(min), α_(m1)}, F_(mid1) ⁽¹⁾≡{α_(m1), α_(m2)},F_(mid2) ⁽¹⁾≡{α_(m2), α_(m3)} and F_(max) ⁽¹⁾≡{α_(m3), α_(max)}. The aimof the embodiment is that the light waves baying the higher incidentangles in the FOV of F_(max) ⁽¹⁾≡{α_(m2), α_(max)} will be reflected byangular sensitive surface 365 and be coupled into the upper substrate360 a, while the light waves having the lower incident angles in the FOVof F_(min) ⁽¹⁾≡{α_(min), α₂} will pass through the angular sensitivesurface 365 and be coupled into the lower substrate 360 b. After beingcoupled into the upper substrate, the light waves having the most higherincident angles in the FOV of F_(max) ⁽¹⁾≡{α_(m3), α_(max)} will bereflected by angular sensitive surface 362 a and be coupled out from thesubstrate, while the second higher incident angles in the FOV ofF_(mid2) ⁽¹⁾≡{α_(m2), α_(m3)} will pass through the angular sensitivesurface 362 a and be coupled out from the substrate by the coupling-outelement 67 a. After being coupled into the lower substrate, the lightwaves having the third higher incident angles in the FOV of F_(mid1)⁽¹⁾≡{α_(m1), α_(m2)} will be reflected by angular sensitive surface 362d and coupled out from the substrate, while the most lower incidentangles in the FOV of F_(min) ⁽¹⁾≡{α_(min), α_(m2)} will pass through theangular sensitive surface 362 b and be coupled out from the substrate bythe coupling-out element 67 b. The main advantage of this option is theoutput aperture is increased by a factor of 4, but disadvantageously,the achievable EMB of the system is actually zero.

An alternative embodiment is to modify the reflection curves of theangular sensitive surfaces 365, 362 a and 362 b, such that thereflection curves as a function of the incident angle, will not fallsharply as did those of surfaces 256 a and 256 b shown in FIGS. 19A and19B. As seen, part of the light waves will be coupled into both theupper and the lower substrates and part of the light waves which arecoupled inside a given substrate 360 i (i=a,b) will be coupled-out byboth the angular sensitive surface 362 i and the coupling-out element 67i. Specifically, part of the light waves in the second highest angularsegment F_(mid2) ⁽¹⁾ will pass through surface 365 and be coupled insidethe lower substrate 360 b, while part of the light wave in the angularsegment F_(mid2) ⁽¹⁾ will be reflected by surface 365 and coupled insidethe upper substrate 360 a. The light waves having the incident angles inthe FOV of F_(up) ⁽¹⁾≡{α_(up), α_(max)} will be coupled inside the uppersubstrate 360 a (wherein α_(up)<α_(m2)), while the light waves havingthe incident angles in the FOV of F_(low) ⁽¹⁾≡{α_(min), α_(low)} will becoupled inside the lower substrate 360 b (wherein α_(low)>α_(low)) As aresult, the light waves having the incident angles in the FOV ofF_(both) ⁽¹⁾≡{α_(up), α_(low)} will be coupled inside the uppersubstrate 360 a as well as the lower substrate 360 b.

Similarly, part of the light waves in the second highest angular segmentF_(mid2) ⁽¹⁾ will be reflected by surface 362 a and coupled out thesubstrate 360 a, while part of the light wave in the angular segmentF_(max) ⁽¹⁾ will pass through surface 362 a and be coupled out thesubstrate 360 a by the coupling-out element 67 a. In addition, part ofthe light waves in the angular segment F_(min) ⁽¹⁾ will be reflected bysurface 362 b and coupled out the substrate 360 b, while part of thelight wave in the angular segment F_(mid1) ⁽¹⁾ will pass through surface362 b and be coupled out the substrate 360 b by the coupling-out element67 b. By proper design, the output brightness of the coupled-out lightwaves will be moderately attenuated, but the requested EMB of the systemwill be covered by the entire angular range of the output light waves.

Another issue to consider is the maximum achievable FOV of the imagewhich is projected into the viewer's eye, In most of thesubstrate-guided based HMD technologies, either reflective ordiffractive, the light waves are coupled out from the guiding substratesubstantially normal to the major surfaces of the substrate.Consequently, due to the Snell refraction from the substrate theexternal FOV of the image is:

F ^((out)) F ^((in))ν·_(s)  (10)

wherein the FOV inside the substrate is Fun) and the refractive index ofthe substrate is v_(s), The orders of the light waves which are coupledinside the substrate should be strictly separated, namely,

α_(min) ⁽¹⁾=α_(min) ⁽⁰⁾+2·α_(sur1)>α_(max) ⁽⁰⁾,  (11)

Therefore, the internal FOV is limited by the constraint

F ^((in))=α_(max) ⁽⁰⁾−α_(min) ⁽⁰⁾<2·α_(sur1,)   (12)

wherein usually a margin of at least 2 degrees should be kept betweenα_(max) ⁽⁰⁾, and α_(min) ⁽⁰⁾ to confirm the separation between the twoorders. The limitation of Eq. (12) yields for systems wherein therefraction indices of the substrate, the coupling-in and thecoupling-out elements are equal.

Referring to FIGS. 24A and 24B, the substrates 360 a and 360 b, thecoupling-in prism 367, and the redirecting prism 80 are all fabricatedfrom the same optical material, and as a result, the two marginal rays,371 and 372, coupled inside the element 355, are refracted intodifferent directions only when passing through the input surface 363 andthe output surface 361 a of the system. As a result of the similaritiesbetween the optical materials which compose element 355, the coupledrays do not experience any refraction when passing through the interfacesurfaces 369, 81 between the substrates and the coupling-in 367 and theredirecting 80 prisms, respectively. Since the optical rays arerefracted only at angles with close proximity to the normal of theentrance 363 and the exit 361 a surfaces, the directions of the rays aremodified according to the approximated equation:

$\begin{matrix}{{\alpha_{out} \sim {\frac{v_{in}}{v_{out}} \cdot \alpha_{in}}},} & (13)\end{matrix}$

and subsequently, the limitation of Eq. (10) is sustained.

The fact that the optical rays enter the substrate at highly obliqueangles can be exploited to improve the above limitation. As illustratedin FIG. 24B, the coupling-in 367 and the redirecting 80 prisms asfabricated from the same optical material having refractive index whichhave the following optical characteristics

ν_(p)<ν_(s) ; A _(p) ˜A _(s)  (14)

wherein ν_(p) is the refractive index of the prisms 367 and 80, andA_(p), A_(s) are the Abbe numbers of the prisms and the substratesrespectively. As a result of the dissimilarities between the opticalmaterial of the substrates 360 a, 360 a and that of the coupling-in 367and the redirecting prisms 80, and the high obliquity that rays 371 and372 incident at the interface surfaces 369 and 81, the rays currentlyexperience substantial refraction when passing through the interfacesurfaces 369 and 81. Since prisms 367 and 80 have the same opticalcharacteristics, the refractions at surfaces 369 and 81 for each passingray will have the same magnitude and the opposite directionsrespectively, and therefore, they will be mutually compensated. Theangular deviation between two different light rays inside the prisms asa function of the deviation inside the substrates can be calculatedaccording to the approximated equation

$\begin{matrix}{{{\Delta\;\alpha_{p}} \sim {{\frac{v_{s}}{v_{p}} \cdot \frac{\cos\;\alpha_{s}}{\cos\;\alpha_{p}} \cdot \Delta}\;\alpha_{s}}},} & (15)\end{matrix}$

wherein α_(s) and α_(p) are the off-axis angles inside the substrate andthe prisms, respectively. Similarly, the angular deviation between therays outside of element 355 is

Δα_(out)˜ν_(p)·Δα_(p).  (16)

Consequently, the ratio between the angular deviation outside element355 and inside the substrates 360 a and 360 b is

$\begin{matrix}{{{\Delta\;\alpha_{out}} \sim {{v_{s} \cdot \frac{\cos\;\alpha_{s}}{\cos\;\alpha_{p}} \cdot \Delta}\;\alpha_{s}}},{or}} & (17) \\{F^{({out})} \sim {F^{({in})} \cdot v_{s} \cdot {\frac{\cos\;\alpha_{s}}{\cos\;\alpha_{p}}.}}} & (18)\end{matrix}$

That is to say, by modifying the optical material of the prisms 369 and80, it is possible to increase the FOV of the system in the air by afactor of

$\frac{\cos\;\alpha_{s}}{\cos\;\alpha_{p}}.$

It should be noted that in order to keep the constraint of Eq. (12), thelight waves having the incident angles of α_(in) ⁽¹⁾<α_(up) should notbe coupled inside the upper substrate 360 a, and the light waves havingthe incident angles of α_(in) ⁽¹⁾>α_(low) should not be coupled insidethe lower substrate 360 b. The first requirement can be achieved byconstructing the interface surface 369 as an angular sensitivereflecting surface, which will be substantially total reflective for theangular range of {α_(up), α_(max)}, and substantially transparent forthe angular range of {α_(min), α_(up)}, in a similar manner to theconstruction of surfaces 256 a and 256 b, as illustrated in relation toFIGS. 19A and 19B. As a result, all the light waves in the angular rangeof {α_(min), α_(up)} which will be reflected by surface 365 will becoupled-out from the upper substrate through surface 369 into the prism367 and blocked by surface 379 of prism 367. The second requirementcould be achieved by causing that all the light waves in the angularrange of {α_(low), α_(max)} to be totally internally reflected bysurface 365, and hence, be coupled inside only the upper substrate 360a.

It should be further noted here that for the most of the relevantdisplay systems, the two requirements should be fulfilled over theentire photopic region. As mentioned with regard to the fabricationprocess of the angular sensitive surfaces 256 a and 256 b, it is usuallyrequired that the Abbe numbers of the optical adhesive, which isadjacent to the surface and the optical material of the substrate, willbe similar to avoid undesired chromatic effects in the image. There arecases, however, wherein the Abbe numbers of the adhesive and the opticalmaterial are substantially different. The chromatic dispersion due tothe variation between the Abbe numbers can be compensated by choosing anoptical material for the coupling-in and the redirecting of prisms 367and 80, having an Abbe number which is different than that of thesubstrates 360 a and 360 b. By proper selection, the difference betweenthe Abbe numbers can induce a chromatic dispersion having the samemagnitude and opposite direction. As a result, the two induceddispersions will be mutually compensated.

The implementation of the angular sensitive reflecting surfaces 362 a,362 b and 365 utilized in the embodiment of FIG. 24B illustrated hereinwith an optical system having the following parameters for substrate 360a:

α_(sur1)=α_(sur2)=8°; F ⁽⁰⁾={38°, 50°}; F ⁽¹⁾={54°, 66°}

F ⁽²⁾={70°, 82°}; α_(sur) ⁽¹⁾={46°, 58°}; α_(sur) ⁽²⁾={62°, 74°},   (19)

and the following parameters for substrate 360 b:

α_(sur1)=α_(sur2)=10.5°; F ⁽⁰⁾={31°, 43°}; F ⁽¹⁾{52°, 64°}

F ⁽²⁾={73°, 85°}; α_(sur) ⁽¹⁾={41.5°, 53.5°}; α_(sur) ⁽²⁾={62.5°,74.5°}.  (20)

The light waves are non-polarized. The optical material of thesubstrates 360 a and 360 b is advantageously Ohara S-LAH88 having arefractive index of v_(d)=1.917, an Abbe number of 30.6, and the opticalmaterial of the prisms 367 and 80 is Ohara S-FTM-88 having a refractiveindex of v_(d)=1.592 and an Abbe number of 35.3. The optical adhesiveswhich are adjacent to surfaces 369, 365, 368 (and 81), 362 a and 362 bare NOA 142, NOA 76, NOA 148, Noa 170 and NOA 61, having refractiveindices of n_(s)=1.42, 1.51, 1.48, 1.70 and 1.56, respectively. Theoverall FOV of the coupled-in image inside the substrates 360 a and 360b is F⁽⁰⁾={31°, 50°}. The overall FOV of the coupled-in image inside theprisms 80 and 367 is F⁽⁰⁾{38°, 32 67°}, and the overall FOV of thecoupled-in image in the air is F⁽⁰⁾={−23°, 23°}, namely, the system hasan FOV of 46° along a single axis. The thickness of each substrate is 1mm and the active area of the output and the input apertures are 25.5 mmand 6.5 mm, respectively.

The FOV in the air, as seen by the viewer, is expanded by a factor of2.4 as compared to the combined FOV inside the substrates 360 a and 360b, and therefore, the limitation given in Eq. (10) is overcome. Byutilizing the expansion illustrated in FIG. 24B, wherein the refractiveindex of prisms 367 and 80 is substantially smaller than that of thesubstrates 360 a and 360 b, it is possible to materialize a systemwherein the ratio between the external FOV in the air and the FOV insidethe substrate is significantly bigger than the refractive index of thesubstrate. This expansion can be exploited not only for the specificconfiguration illustrated in FIG. 24B, but generally for any substrate,even with a single coupling-out element, having optical materialdifferent than the coupling-in and the redirecting prisms.

FIGS. 25A to 25C illustrate the reflectance curves of the angularsensitive reflecting surfaces 362 a, 362 b and 365 as the function ofthe normalized FOV inside the substrates.

FIG. 26 illustrates the efficiency of element 355 as a function of theFOV. As shown, in most of the FOVs the efficiency is between 45% and50%, while at the edges of the FOVs the efficiency is substantiallyhigher. Even though the output aperture is expanded by a factor of four,the efficiency is attenuated only by a factor of two. Regarding thenon-uniformity of the efficiency at the edges of the FOV, for most ofthe back and front illuminated displays such as LCD and LCOS, theillumination, and hence, the brightness of the display sources, areusually stronger at the center of the display as a result of theGaussian distribution of the illuminating light source. Consequently,the non-uniform efficiency curve of the system can compensate for thenon-uniform illumination, and in addition, the brightness of thecoupled-out image can be improved. For display sources having a uniformbrightness distribution, it is possible to attenuate the higherintensities at the edges of the image either electronically or opticallyby reducing the reflectance of surface 67 b of FIG. 24B for light wavesat the left edge of the FOV and by reducing the transmission of surface81 for light waves at the right edge of the FOV.

The expansion can be exploited not only for the specific configurationillustrated in FIG. 24B, but generally for any substrate. As illustratedin FIG. 27A, this expansion is utilized for a system having a singlesubstrate 64 and a single coupling-out surface 67, wherein the twomarginal rays 382 and 383 are coupled into the substrate through acoupling-in prism 367 and are redirected into the viewer's eye 24 by aredirecting prism 80, wherein the substrate 64 has an optical materialdifferent than the coupling-in and the redirecting prisms. FIG. 27Billustrates an embodiment wherein the marginal rays 386 and 389 arecoupled into the substrate by reflection from an external surface 363 ofthe coupling-in prism 367.

The non-uniformity of the projected image into the viewer's eye will bereferred to with reference to FIG. 28A. Three different rays, 331, 332and 333 a are coupled-out from the substrate 64 by coupling-out element67 and then redirected by the pallidly reflecting surfaces 79 a and 79 binto the viewer's eye. On the other hand, ray 333 b pass through surface79 b before being reflected by surface 79 a. As a result, the brightnessof ray 333 b is attenuated by surface 79 b, as a function of thetransmittivity of that surface, before reaching the viewer's eye. Asillustrated in FIG. 28B, which plots the normalized brightness of thecoupled-out light waves as a function of the lateral coordinate x of theoutput aperture 89, the output brightness is reduced at a central partof the output aperture. This brightness pattern has the form of a darkstripe over a bright background, or, for an array of several partiallyreflecting surfaces 79 a, 79 b, 79 c, . . . , a pattern of alternatingdark and bright snipes, not shown. For near-to-eye displays, the eyeintegrates the light wave emerging from a single viewing angle andfocuses it onto one point on the retina, and since the response curve ofthe eye is logarithmic, small variations, if any, in the brightness ofthe display, will not be noticeable. Therefore, if the stripes are denseenough (namely, the lateral dimension of each stripe is significantlysmaller than the eye's pupil), and if the eye is positioned close enoughto the substrate, the viewer can still experience a high-quality imageeven with the stripes, For displays which are located at a distance fromthe eye, however, such as head-up displays (HUDs), the stripes will benoticeably seen by the viewer's eyes, which significantly reduces imagequality and the overall performance of the optical system. Therefore, asolution must be found to the stripes phenomenon to allow theexploitation of the projection elements, proposed in this application,for far-from-eyes applications.

There are several different alternatives for achieving the HUDconfiguration. As illustrated in FIG. 29A, a collimated image 322 a, 322b is coupled into the substrates 310 a, 310 b from the side part of theviewer's head and is propagating inside the substrate along thehorizontal axis, in that case, exit pupil of the optical system 255 a isdetermined by both eyes of the viewer, and hence, is extendedsignificantly compared to an EMB of a near-to eye system, wherein theimage is projected into a single eye of a viewer. Consequently, theoutput aperture of the optical system should be increased accordingly, asingle coupling-out element is not enough and at least a few differentcoupling-out elements should be utilized to accomplish the requiredaperture.

FIG. 29A illustrates a possible embodiment wherein four coupling-outelements, 256 a, 256 b, 67 a and 67 b, are utilized according to theembodiment in relation to FIG. 17. The problem with the dark stripesoccurs in this embodiment: while the light rays 322 ba and 322 bb arecoupled out from the substrates 310 a and 310 b by the coupling-outelements 67 a and 256 b, respectively, and then redirected by thepartially reflecting surfaces 79 d and 79 b into the viewer's eyes, andray 322 ba is attenuated by surface 79 b before being reflected bysurface 79 c into the viewer's eyes. A possible solution for thenon-uniformity problem is to utilize a single partially reflectingsurface 79 (as illustrated. FIG. 5A) instead of an array of surfaces.The result of utilizing this solution, however, is that the thickness ofthe prism 80, and consequently the size of the entire optical system,will be increased beyond the point of a reasonable use of the system.Another possible solution is to utilize a dense array of surfaceswherein the lateral dimension of each element is in the order of 1-2 mm.As a result, the lateral dimension of the partially blocked segment ineach element will be in the order of 0.1-0.2 mm and will be unnoticeableat a watching distance of 30-50 cm.

An alternative solution to the non-uniformity problem, exploitingdynamically controlled partially reflecting surfaces, is illustrated inFIGS. 29B and 29C. It is assumed that the dynamic surfaces 79 i (i=a,b,. . . ) are operated as a totally reflective p section (p<1) of eachframe-time to yield a system having a reflectivity of p and atransmissivity of (1−p). Instead of operating the reflectance of all thedynamic surfaces simultaneously, it is possible to operate themalternately, namely, the surfaces having an odd index and those havingan even index are activated as reflective surfaces at two exclusivelyseparated operating times during each frame-time. Since each surface ispartially blocked only by its right adjacent surface, and two adjacentsurfaces are not activated simultaneously any more, the partial blockingproblem is avoided. As illustrated in FIG. 29B, only surfaces 79 a, 79 cand 79 c are activated, during the first period of time p, as fullyreflective, while surfaces 79 b and 79 d are fully transparent. As aresult, ray 322 ba is no longer blocked by surface 79 b and isredirected, after being coupled-out by surface 256 b, into the viewer'seyes. As illustrated in FIG. 22C, only surfaces 79 b and 79 d areactivated, during the second period of time p, as fully reflective whilesurfaces 79 a, 79 c and 79 e, are fully transparent. Now, the light rays322 a and 322 bb are coupled out from the substrates 310 a and 310 b bythe coupling-out elements 67 a and 256 b, respectively, and thenredirected by the active reflective surfaces 79 d and 79 b into theviewer's eyes, while ray 322 ba passes through the transparent surface79 c. With this proposed embodiment, different rays reach the viewer'seye at different time slots. All of these time slots, however, arecontained in the same frame-time, for all the light rays of the image.Therefore, because of the persistence of vision, the light rays from allthe pixels of the display source will be integrated into the viewer'seye, thereby creating a single image. Since the two sets of surfaces arealternately operated at two distinct time periods p during a single timeframe, the value of p should satisfy the condition

p<0.5.  (21)

As a result, the efficiency of the projected image into the viewer'seyes cannot exceed the value of 50%.

An alternate configuration for the HUD embodiment, wherein the displaysource and the collimating module 360 are positioned at the lower (orthe upper) part of the optical module and the coupled light waves arepropagating along the vertical axis, is illustrated in FIGS. 30A to 30B.Here, the exit pupil is extended along the normal axis to thepropagation direction inside the substrate and the aperture should beextended accordingly. The non-uniformity issue can also be solved heresimilarly to the manner illustrated with reference to FIGS. 29A to 29C,but an even simpler solution can be utilized here. Unlike theembodiments of near-to-eye displays, wherein the combiner should besubstantially normal to the line-of-sight of the viewer, for the HUDconfigurations, it is possible to rotate the combiner around thehorizontal axis (or another axis which is normal to the line of sight ofthe viewer) at a substantial angle, which is usually around 450.

As illustrated in FIG. 31A, it is possible to design the configurationof the coupling-in and the coupling-out element, such that the centrallight wave of the image (that is, the light wave which is located at thecenter of the FOV of the image), after being coupled-out by thecoupling-out element, is refracted from the substrate at a directionwhich is oriented at a substantial off-axis angle which is nearly 45°relative to the major surfaces of the substrate. As a result, since thesubstrate is rotated at an approximate angle of 45°, the partiallyreflecting surface 79 should not be rotated with respect to the majorsurface to redirect the light waves into the viewer's eyes. Element 79can be a single fiat partially reflecting element which is attached tothe major surface 72 of the substrate. Consequently, the light waveswhich are coupled out from the substrate are redirected into theviewer's eye by a flat uniform surface and the non-uniformity issue istotally aborted.

FIGS. 31A to 31C illustrate a possible embodiment wherein fourcoupling-out elements, 256 a, 256 b, 67 a and 67 b, are utilized toexpend the output aperture along the vertical axis according to theaforementioned embodiments in relation to FIGS. 17A to 17C. Seen in FIG.31A are two rays 420 a and 420 b from the same plane input wave havingincident angles of α_(m1)<α_(si) ⁽⁰⁾<α_(m2), which impinge on theangular sensitive coupling-out elements 256 a and 256 b, respectively.Ray 420 a is reflected once from the coupling-in element 55 a, trappedinside the substrate 410 a at an off-axis angle of α_(in) ⁽¹⁾=α_(in)⁽⁰⁾+2·α_(sur1), and impinges on the right part of surface 256 a at anincident angle of α_(si) ⁽¹⁾=α_(in) ⁽¹⁾=α_(sur1). As a result ofcondition (c) described with reference to FIGS. 17A to 17C hereinabove,ray 420 a passes through surfaces 256 a and after a single reflectionfrom the left side of surface 256 a, it continues to propagate insidesubstrate 410 a at an off-axis angle of α_(in) ⁽²⁾, and after a doublereflection from surface 67 a is coupled-out from substrate 410 a, andredirected into the viewer's eye by the flat partially reflectingsurface 79. Ray 420 b is reflected once from the coupling-in element 65b, trapped inside the substrate 410 b at an off-axis angle of α_(in)⁽¹⁾=α_(in) ⁽⁰⁾+2·α_(sur1), and impinges on the right part of surface 256b at an incident angle of α_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). As a result ofcondition (c) described hereinabove, ray 420 b is reflected from surface256 b, coupled-out from the substrate 410 b and is redirected by theflat partially reflecting surface 79 into the viewer's eye, wherein theparallel rays 420 a and 420 b propagate substantially co-linear to theviewer's line-of-sight.

FIG. 31B illustrates two rays 421 a and 421 b from the same plane inputwave having incident angles of α_(si) ⁽¹⁾<α_(m1), which impinge on theangular sensitive coupling-out elements 256 a and 256 b, respectively.As a result of condition (b) described hereinabove with reference toFIGS. 17A to 17C, both rays pass through surfaces 256 a and 256 b. Ray421 a is reflected twice from the coupling-in element 65 a, trappedinside the substrate 410 a at an off-axis angle of α_(in) ⁽²⁾=α_(in)⁽⁰⁾+4·α_(sur1), and reflected once from surface 256 a before beingimpinged on the left part of surface 256 a at an incident angle ofα_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). Ray 421 b is reflected once from thecoupling-in element 65 b, trapped inside the substrate 410 b at anoff-axis angle of α_(in) ⁽²⁾=α_(in) ⁽⁰⁾+2·α_(sur1), and impinges on theleft part of surface 256 b at an incident angle of α_(si) ⁽¹⁾=α_(in)⁽¹⁾−α_(sur1). After passing through surface 256 a, ray 421 a continuesto propagate inside substrate 410 a at an off-axis angle of α_(in) ⁽¹⁾,and after a single reflection from surface 67 a is coupled-out fromsubstrate 410 a and redirected into the viewer's eye by the flatpartially reflecting surface 79. After passing through surface 256 b,ray 421 b is reflected once from the left side of surface 256 b,continues to propagate inside substrate 410 b at an off-axis angle ofα_(in) ⁽²⁾, and after a double reflection from surface 67 b iscoupled-out from substrate 410 b, and redirected into the viewer's eyeby the flat partially reflecting surface 79.

FIG. 31C illustrates two rays 422 a and 422 b from the same plane inputwave having incident angles of α_(si) ⁽⁰⁾>α_(m2) a which impinge on theangular sensitive coupling-out elements 256 a and 256 b, respectively.As a result of condition (a) described hereinabove with reference toFIGS. 17A to 17C, both rays are reflected from surfaces 256 a and 256 band are coupled-out from the substrates 410 a and 410 b by these angularsensitive reflective surfaces, respectively. Ray 422 a is reflected oncefrom the coupling-in element 65 a, trapped inside the substrate 410 a atan off-axis angle of α_(in) ⁽¹⁾=α_(in) ⁽⁰⁾+2·α_(sur1), and impinges onthe right part of surface 256 a at an incident angle of α_(si)⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1). Ray 422 b is reflected twice from thecoupling-in element 65 b, trapped inside the substrate 410 b at anoff-axis angle of α_(in) ⁽²⁾=α_(in) ⁽⁰⁾+4·α_(sur1), and is reflectedonce from surface 256 b before being impinged on the left part ofsurface 256 b at an incident angle of α_(si) ⁽¹⁾=α_(in) ⁽¹⁾−α_(sur1).After being reflected and coupled-out from the substrates, rays 422 aand 422 b are redirected by the flat partially reflecting surface 79into the viewer's eye.

The implementation of the rotated HUD system utilized in the embodimentsof FIGS. 31A to 31C is illustrated herein with an optical system havingthe following parameters:

α_(sur1)=α_(sur2)=12°; F ⁽⁰⁾={18°, 28°}; F ⁽¹⁾={42°, 52°}

F ⁽²⁾={66°, 76°}; F _(sur1) ⁽¹⁾={30°, 40°}; F _(sur1) ⁽²⁾={54°,64°},  (22)

The light waves are s-polarized and the system has a quasi-monochromaticdisplay source having a wavelength of λ=550 nm. The optical material ofthe substrate 64 is Schott N-SF57 having a refractive index ofν_(d)=1.846, and the optical adhesives which are adjacent to surfaces256 a and 256 b is NOA 1315, having refractive indices of ν_(d)=1.315.The overall FOV of the coupled-in image is F⁽⁰⁾={18°, 28°} (which ispractically an FOV of F^((air))={35°, 61°} in the air). The angularrange F_(sur1) ⁽⁰⁾≡{30°, 40°} is divided into three substantially equalsegments: F_(low) ⁽⁰⁾≡{30°, 33.33°}, F_(mid) ⁽⁰⁾≡{33.3°, 36.7°} andF_(max) ⁽⁰⁾≡{36.7°, 40°}. The optical materials which yield criticalangles of 33.3° and 36.7° for a substrate having a refractive index ofν_(d)=1.846 should be 1.013 and 1.103, respectively. Optical materialhaving the required optical indices cannot be practically found.Therefore, for optical system using quasi-monochromatic light it ispossible to “shift” leftward the reflectance curve of the angularsensitive surface utilizing a proper dielectric coating.

FIG. 32A illustrates a graph of the reflection from the angularsensitive reflective is surface 256 a in FIGS. 31A to 31C, coated withan appropriate dielectric coating as a function of the incident anglefor the wavelength of 550. As shown, the reflection is substantially100%, even below the critical angle of 45□, for angular spectrum above37°, while it is very low for the incident angles of {30°, 36.4°}. FIG.25B illustrates the graph of the reflection from the angular sensitivereflective surface 256 b in FIGS. 24A to 24C coated with an appropriatedielectric coating as a function of the incident angle for thewavelength of 550. As shown, the reflection is 100%, even below thecritical angle of 45□, for angular spectrum above 33.7°, while it isvery low for the incident angles of {30°, 33°}.

FIG. 33 illustrates two marginal and central light waves of the imagewhich are coupled out from the substrate and redirected into theviewer's eyes 24R and 24L. As shown, the light waves 420, 421 and 422,having the zero order off-axis angles of α_(in) ⁽⁰⁾(min), α_(in)⁽⁰⁾(max) and α_(in) ⁽⁰⁾(mid), are illuminating each only the parts ofthe flat partially reflecting surface 79 which are required toilluminate the EMB. That is to say, the rightmost light wave 422(dashed-dotted lines) is reflected only from the right part of surface79, the leftmost light wave 421 (dashed lines) is reflected only fromthe left part of surface 79, while the central light wave 420 (dottedlines) is reflected from the central part of surface 79. All the lightwaves reach the viewer's eyes over the entire extent of the HMB 297.Consequently, the HMB 297 of the embodiment illustrated in FIG. 33 hasbeen extended without decreasing the image's brightness, and when thenon-uniformity issue is entirely resolved. The brightness of thecoupled-out light waves can be increased even furthermore by exploitinga dynamically controlled partially reflecting surface 79. The maximalefficiency of the dynamic surface 79 is not currently limited to 50% asin the embodiment illustrated in FIG. 29, and it can be practicallyincreased to nearly 100%. That is to say, the brightness of thecoupled-out image waves which reaches the viewer's eyes could be closeto the brightness of the light waves coupled into the substrate.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

In particular it should be noted that features that are described withreference to one or more embodiments are described by way of examplerather than by way of limitation to those embodiments. Thus, unlessstated otherwise or unless particular combinations are clearlyinadmissible, optional features that are described with reference toonly some embodiments are assumed to be likewise applicable to all otherembodiments also.

What is claimed is:
 1. An optical device, comprising: alight-transmitting substrate having at least two parallel majorsurfaces, edges and an output aperture; an optical element for couplinglight waves into the substrate to effect total internal reflection; aplurality of redirecting elements positioned outside of the substratecomprising at least two spaced-apart redirecting elements having aselectable laterally displaceable reflection-transmission ratio, and atleast one reflecting surface having at least one active side locatedbetween the two major surfaces of the light-transmitting substrate forcoupling light waves out of the substrate, wherein light waves trappedinside the substrate are coupled out from the substrate through theoutput aperture substantially inclined in relation to the normal to thesubstrate major surfaces, and are reflected from the redirectingelements into a viewer's eye, and wherein at least one of theredirecting elements is laterally displaceable in relation to anotherredirecting element.
 2. The optical device according to claim 1, whereinthe reflection-transmission ratio of the redirecting elements iscontrolled by the lateral displacement of the spaced-apart redirectingelements.
 3. The optical device according to claim 2, wherein each ofthe redirecting elements comprises an array of reflecting elementscharacterized by a fill-factor.
 4. The optical device according to claim3, wherein the fill-factor is not higher than 50%.
 5. The optical deviceaccording to claim 3, wherein the fill-factor is substantially 1/n, nbeing the number of the redirecting elements.
 6. The optical deviceaccording to claim 1, wherein the reflection-transmission ratio has avalue between 1/n:(n−1)/n and 100%:0%.
 7. The optical device accordingto claim 1, wherein each of the redirecting elements comprises atwo-dimensional array of sub-mirrors.
 8. The optical device according toclaim 7, further comprising a control unit, wherein for at least one ofthe redirecting elements each sub-mirror is separately translated by thecontrol unit.
 9. The optical device according to claim 8, furthercomprising a piezoelectric device, wherein the sub-mirrors can betranslated using the piezoelectric device.
 10. The optical deviceaccording to claim 1, wherein the optical element for coupling lightwaves into the substrate is a second reflecting surface carried on aslanted edge of the substrate and is parallel to the reflecting surface.11. The optical device according to claim 1, wherein the light wavesreflected by the redirecting elements pass through the substrate. 12.The optical device according to claim 11, wherein the light wavesreflected by the redirecting elements substantially pass through thesubstrate with no significant reflections.
 13. The optical deviceaccording to claim 2, wherein the lateral displacement of thespaced-apart redirecting elements is dynamically controlled.
 14. Theoptical device according to claim 2, wherein the lateral displacement ofthe spaced-apart redirecting elements is manually controlled by theviewer.
 15. The optical device according to claim 2, further comprisinga photometer measuring the brightness of the external scene, wherein thelateral displacement of the spaced-apart redirecting elements is setaccording to the brightness detected by the photometer.